fchdir: fix logic bugs

fchdir: fix logic bugs

Configuring with ac_cv_func_fchdir=no on a system that has fchdir
and where open handles directories, just to test out the replacement
capabilities, uncovered an m4 test bug and a link failure on rpl_fstat.

* m4/fchdir.m4 (gl_FUNC_FCHDIR): Fix logic bug.
* tests/test-fchdir.c (main): Enhance test.
* lib/fchdir.c (rpl_fstat): Always provide if fchdir replacement
is in use.

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