This patch does three things:

* Update `flycheck-default-executable-find` to search remote hosts when
  `default-directory` is a remote directory. See also the documentation for
  [executable-find](https://www.gnu.org/software/emacs/manual/html_node/elisp/Locating-Files.html).

* Change uses of `call-process` and `start-process` to `start-file-process`
  which is the same as these two functions, but starts the process on a remote
  host when `default-directory` is a remote directory. See also the
  documentation for
  [start-file-process](https://www.gnu.org/software/emacs/manual/html_node/elisp/Asynchronous-Processes.html).

* Passes the local part of `buffer-file-name` to the compile command,
  e.g. strips `/ssh:10.0.0.42`, since the checker is run on the remote machine
  where the file name must be local. See also the documentation for
  [file-local-name](https://www.gnu.org/software/emacs/manual/html_node/elisp/Magic-File-Names.html).

Such checkers (including python-pycompile) use a temp directory.

For executing flycheck on remote files, temp directories should be
created on remote.

Some checkers (including python-flake8 and python-pycompile) use
flycheck-parse-error-with-patterns as their error-parser.

This commit changes `flycheck-parse-error-with-patterns' behavior so
that it always returns `flycheck-error' struct with its :filename slot
in the form of local path on remote files (i.e., without
"/method:user@host:" part).

Some checkers set remote path or filename (without directory path) to
`flycheck-error' struct's :filename slot.

For example, on remote files,
python-pylint sets remote path (i.e., with "/method:user@host:" part).
python-flake8 before the previous comiit also sets remote path.
py-compile sets filename only.

In all these cases, after processing with
`flycheck-fill-and-expand-error-file-names', :filename slot of
`flycheck-error' struct was remote path.

This commit changes `flycheck-fill-and-expand-error-file-names'
behavior so that it always returns `flycheck-error' struct with its
:filename slot in the form of local path on remote files (i.e.,
without "/method:user@host:" part).

Some checkers (including python-pylint) use
`flycheck-save-buffer-to-temp' to create temp file.

For executing flycheck on remote files, their corresponding temp files
should be created on remote (therefore, they should be specified as
remote path, i.e., with "/method:user@host:" part).
On the other hand, the return value of `flycheck-save-buffer-to-temp'
should be local path (i.e., without "/method:user@host:" part) because
the value is used as an argument to checker's executable.

Flycheck uses `flycheck-locate-config-file-functions' to locate config
file. Its value is the list of functions below:
- flycheck-locate-config-file-by-path
- flycheck-locate-config-file-ancestor-directories
- flycheck-locate-config-file-home

Each locate function should use remote path (i.e., with
"/method:user@host:" part) to locate a config file.

When the return value is used as an argument to a checker's
executable, the path should be converted to local path (i.e., without
"/method:user@host:" part).

Tested with python-flake8 and python-pylint.

The original version used stdin instead of a temporary file (as with
javascript-eslint). However, behavior of eslint and stylelint is
different in terms of stdin processing.

Corresponding CLI commands for both tools are:
eslint:
eslint --format=json --stdin --stdin-filename FILENAME-TO-ASSIGN-TO-STDIN

stylelint:
stylelint --formatter json --stdin-filename FILENAME-TO-ASSIGN-TO-STDIN
or
stylelint --formatter json --stdin --stdin-filename FILENAME-TO-ASSIGN-TO-STDIN

eslint waits for stdin EOF and keeps itself alive, while stylelint
does not wait and may exit before input comes in.

When stylelint does not receive stdin,
it emits its usage message (when --stdin is not specified)
or outputs "empty source" lint message (when --stdin is specified).

For local files, the original version was able to pass input with
`flycheck-process-send-buffer' before stylelint process exited.
For remote files, stylelint process exited before
`flycheck-process-send-buffer' called.

This commit changes *-styelint commands so that they use a temporary
file (instead of stdin) as their input to avoid the above situation on
remote.

To add support for remote files, some occurrences of filenames had to
be converted to local path on remote (i.e., without
"/method:user@host:" part).
However, these filenames could be nil, while file-local-name signals
an error when its argument is nil.

This led to test failure for scss-lint and others.
(Test command: "make LANGUAGE=scss integ")

This commit adds a wrapper function of file-local-name that accepts
nil as its argument and return it as it is.

Replace all occurrences is excessive.
For example, arguments to checker executables are not expected to be
nil and thus file-local-name will suffice.
However, here, all occurrences are replaced just in case.

In `flycheck-locate-config-file-home', file-remote-p is used to
determine if the context is local or remote.
Because file-remote-p does not accept nil,
(file-remote-p (buffer-file-name))
may cause an error.

It led to unit test failure:
   FAILED  flycheck-locate-config-file/existing-file-in-home-directory
   FAILED  flycheck-locate-config-file/existing-file-in-parent-directory
   FAILED  flycheck-locate-config-file/not-existing-file

This commit adds a nil-check for buffer-file-name.
When buffer-file-name returns nil, the context is assumed to be local.

integ test for Emacs25.3 failed due to undefined functions.

Error: the following functions are not known to be defined:
file-local-name, temporary-file-directory

This commit fixes the problem by reverting to the original codes when
the functions are not available.

The outputs of flake8 are parsed by flycheck-parse-with-patterns, which does
not take ASCII escape sequences into account.
Therefore, flake8 should return simple texts (without escape sequences).