1. Do not ask for the same option value twice.

2. For tty, we always want false, unless specified, and this is what
   GetBool gets us.

Signed-off-by: Kir Kolyshkin <[email protected]>

An error from strconv.Atoi already contains the text it fails to parse.
Because of that, errors look way too verbose, e.g.:

	[root@kir-rhat runc-tst]# ./runc exec --user 1:1:1 2345 true
	ERRO[0000] exec failed: parsing 1:1 as int for gid failed: strconv.Atoi: parsing "1:1": invalid syntax

With this patch, the error looks like this now:

	[root@kir-rhat runc]# ./runc exec --user 1:1:1 2345 true
	ERRO[0000] exec failed: bad gid: strconv.Atoi: parsing "1:1": invalid syntax

Still not awesome, but better.

Signed-off-by: Kir Kolyshkin <[email protected]>

Switch from github.com/moby/sys/user to Go stdlib os/user
(which has both libc-backed and pure Go implementations).

Signed-off-by: Kir Kolyshkin <[email protected]>

OCI runtime spec states [1] that the UID, primary GID, and additional
GIDs are all specified as numbers, and also adds that symbolic names
resolution "are left to upper levels to derive". Meaning, runc should
not care about user and group names.

Yet, runc tries to be clever than that, always parsing container's
/etc/passwd and /etc/group. It results in a few things:

1. If UID (or GID) specified can't be found inside container's /etc/passwd
   (or /etc/group), runc (run or exec) errors out.

2. Any additional GIDs specified in container's /etc/group are
   automatically prepended to the list for setgroups(2). Meaning, a user
   can either specify additional GIDs in OCI runtime spec, or
   container's /etc/group entry for a given user.

Looks like (1) is questionable (on a normal Linux system, I can run
programs under any UID (GID), not limited to those listed in /etc/passwd
(/etc/group), and (2) is just an extra mechanism of specifying
additional GIDs.

Let's remove those, hopefully increasing runc performance as well as OCI
spec conformance.

The only remaining need to parse /etc/passwd is to set HOME environment
variable for a specified UID, in case $HOME is not yet set. Use
user.LookupUid for this case.

PS Note that the structures being changed (initConfig and Process) are
never saved to disk as JSON by runc, so there is no compatibility issue
for runc users. Still, this is a breaking change in libcontainer, but we
never promised that libcontainer API will be stable.

For 3998.

[1] https://github.com/opencontainers/runtime-spec/blob/v1.0.2/config.md#posix-platform-user

Signed-off-by: Kir Kolyshkin <[email protected]>