This reverts commit 1bc992bfc3.
We originally thought `podman varlink` was entirely removed, but
that was not true. We originally thought that
`podman system service --varlink` worked the same as
`podman varlink` but that was also not true. `system service` is
broken when used under systemd units, and `podman varlink` still
exists and works. Revert the change to `podman system service` to
fix socket-activated Varlink under systemd.
Signed-off-by: Matthew Heon <mheon@redhat.com>