fchdir: port to mingw

fchdir: port to mingw

* m4/fchdir.m4 (gl_FUNC_FCHDIR): Check for mingw bug.
* lib/open.c (open) [FCHDIR_REPLACEMENT]: If directories can't be
opened, then use a substitute.
* lib/sys_stat.in.h (fstat) [REPLACE_OPEN_DIRECTORY]: Declare
replacement.
* lib/fchdir.c (fstat) [REPLACE_OPEN_DIRECTORY]: Implement it.
(_gl_register_fd): No need to check stat if open already filters
all directories.
(fchdir): Fix error condition to match POSIX.
* modules/fchdir (Depends-on): Add sys_stat.
* doc/posix-functions/open.texi (open): Document the limitation.
* modules/fchdir-tests: New file.
* tests/test-fchdir.c: Likewise.

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