fflush: also replace fclose when fixing fflush

fflush: also replace fclose when fixing fflush

This fixes the fclose failures detected in the previous patch,
but only when the GPL fflush module is also in use.  That is
because the need for behavior of resetting seekable input streams
is much less common, and the fix more complex.  The LGPLv2+ test
for fclose() in isolation is relaxed to pass if fflush is not
being replaced to cater to input streams.

* modules/fflush (Depends-on): Add fclose.
* m4/fflush.m4 (gl_FUNC_FFLUSH): Also replace fclose.
* lib/fclose.c (rpl_fclose): Don't cause spurious failures on
memstreams with no backing fd.
* doc/posix-functions/fclose.texi (fclose): Document the use of
fflush module to fix the bug.
* tests/test-fclose.c (main): Relax test when fclose is used in
isolation.

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