Move service_enable="YES" rc.conf(5) additions to
VM_RC_LIST and deduplicate.

Evaluate if firstboot_freebsd_update should be enabled
based on UNAME_r, because it is not supported for the
-CURRENT or -STABLE branches.

MFC after: 2 weeks
X-MFC-with: r277458, r277536, r277606, r277609,

		r277836

Sponsored by: The FreeBSD Foundation