So far we created the target directory, and the source for bind mounts,
but not workdir/upperdir for overlays, so it has to be done separately
and strictly before the unit is started, which is annoying. Check the
options when creating directories, and if upper/work directories are
specified, create them.
Mount units can do it, but the command line tool cannot, as it needs a
valid 'what'. If --tmpfs/-T if passed, parse the argument as 'where'
and send a literal 'tmpfs' as the 'what' if not specified.
Luca Boccassi
Frantisek Sumsal
Franck Bui
Yu Watanabe