sysutils/podman: Allow setting ownership on auto-created socket

The podman_service daemon auto-creates a socket on startup, along with
parent directory, and is always run as root. It is often useful to have
another proxy like haproxy or nginx provide more sophisticed security,
and these daemons do not need root privileges.

Approved by: dfr
Reported by: pat@patmaddox.com
Tested by: arrowd
Differential Revision: https://reviews.freebsd.org/D55455