PATCH: Support "@file"

[DJ Delorie]

> >>  --extra-switches-from-file=<file>
> 
> > If we do it, I think we should do that in the existing expandargv.  That
> > will ensure it's consistent across all programs which use expandargv,
> > which seems like a good thing.
> 
> Agreed - DJ - do you think that this is a good idea ?  I can submit a 
> patch for exandargv if you like.

I don't see the need, and neither DJGPP nor MSVC support it (they both
support @file).

Plus, @file is for more than just switches - it's for arbitrary
command line arguments, including for example, file names found with
"find".

If you want to play with expandargv, I'd rather the effort go into
supporting find's -print0 option, by detecting NULs in the file and
switching to NUL-delimited arguments instead of whitespace (and
skipping the quoting logic too).  This feature would give us a
security layer for managing files that the usual shell quoting
doesn't.