tac: fall back to /tmp if $TMPDIR is unavailable

tac: fall back to /tmp if $TMPDIR is unavailable

This also refactors temp_stream() to its own module,
in preparation for use by split.

* src/tac.c: Refactor temp_stream() out to ...
* src/temp-stream.c: ... A new module mostly refactored from tac,
but uses tmpdir to more robustly support $TMPDIR,
while falling back to /tmp if not available.
* src/temp-stream.h: The new module interface.
* src/local.mk: Reference the new module from tac.
* tests/tac/tac.pl: Adjust to non failing missing $TMPDIR.
* po/POTFILES.in: Reference the new module with translatable strings.
* NEWS: Mention the user visible improvements to tac TMPDIR handling.