tmpfiles.sh: add support for + modifier

systemd added support for b+, c+, p+, and L+ in 2e78fa79 and 1554afae to
remove the target path if it already exists.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
tmpfiles.sh: add support for C action

Recursively copies files or directories.  Added by systemd in 849958d1.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>