rename, fchdir: don't ignore chdir failure

rename, fchdir: don't ignore chdir failure

Although we just checked that chdir(cwd) worked, there is a
race where it could disappear while we are temporarily away.
If that happens, forcefully give up rather than proceeding
in the wrong directory.

* lib/fchdir.c (get_name): Abort on unexpected chdir failure.
* lib/rename.c (rpl_rename) [W32]: Likewise.
(rpl_rename) [RENAME_DEST_EXISTS_BUG]: Avoid one case of losing
an empty destination directory if source cannot be renamed,
although there is still possibility for failure.
* doc/posix-functions/rename.texi (rename): Document the race.
Reported by Jim Meyering.

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