shared: add file_create_dated() helper

For easy creation of unique new files. I'm looking at you,
screenshooter.

This code is based on timeline.c weston_timeline_do_open().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Daniel Stone <daniels@collabora.com>