fclose: avoid double close race when possible

fclose: avoid double close race when possible

Calling close(fileno(fp)) prior to fclose(fp) is racy in a
multi-threaded application - some other thread could open a new file,
which is then inadvertently closed by the fclose that we thought
should fail with EBADF.  For mingw, this is no worse than the race
already present in close_fd_maybe_socket for calling closesocket()
prior to _close(), but for all other platforms, we might as well be
nice and avoid the race.

* lib/fclose.c (rpl_fclose): Rewrite to avoid double-close race on
all but WINDOWS_SOCKETS.

Signed-off-by: Eric Blake <eblake@redhat.com>