Remove nulls from input

[zenzike]

Fixes #317.

[lcd047]

This breaks on systems that don't have tr, for example Windows.

[zenzike]

Fair enough. I'll take a look at a vim only solution.

[lcd047]

Ideally there would be a postprocess function that would do that. However, Vim truncates the lines at \0 while reading the error files. I'd speculate that's a bug in Vim, but since Vim development process is what it is, that information is probably less than useful. :)

As @eagletmt points out, readfile() is one of the few functions in Vim that does cope with \0. A possible solution would be to call system() instead of lmake, preprocess the error file, and parse errors with lgetexpr. The preprocess part might be something like applying a (list of) function(s) to each line in the error files, more or less like the existing postprocess functions. That would allow us to get rid of some helper scripts (f.i. validator_decode.awk), and make the error files more manageable before parsing them. The problem is writing a system() incantation that works the exact same way as lmake, on all systems.