fchdir: another mingw fix

fchdir: another mingw fix

canonicalize_file_name does not understand drive letters or
backslash.  The only reason openat required it was to make
fchdir get the canonical name of a directory.  But we can do
the same trick with chdir and getcwd.  With this fix,
fchdir(open("..",O_RDONLY)) finally does the right thing on mingw.

* modules/fchdir (Depends-on): Drop canonicalize-lgpl.
* lib/fchdir.c (get_name): New helper method; skips canonicalize
on mingw (where it has not yet been ported), and make it optional
elsewhere.
(_gl_register_fd): Use it.

Signed-off-by: Eric Blake <ebb9@byu.net>