cow, cache: Better mkostemp fallback

cow, cache: Better mkostemp fallback

In 2018, we added fallbacks for Haiku lacking mkostemp (one of the
atomic CLOEXEC interfaces proposed for future POSIX [1]); however,
that fallback has minor bugs: if mkstemp() fails, we blindly call
fcntl(-1) (which probably changes the errno later reported against
"mkostemp: %s: %m"); and although it is historically unlikely that any
other bits will be set, F_SETFD should be used in a read-modify-write
pattern rather than a blind overwrite pattern.

[1] http://austingroupbugs.net/view.php?id=411

Update these fallbacks to use our newly-added set_cloexec utility
function; neither place needs atomicity because they occur during
.load (where we are still more-or-less single-threaded), rather than
while another thread may be actively inside a plugin (such as sh)
using fork().

Fixes: 60076fbcfd
Fixes: b962272a56
Signed-off-by: Eric Blake <eblake@redhat.com>