ifconfig.8: cleanup reminiscence about long gone ppp(4)

Replace ppp(4) removed since FreeBSD 8.0-RELEASE with vlan(4).
While here, remove commented out reference to non-existing "egress"
interface group hiding since initial import of interface groups
from OpenBSD in 2006.

(cherry picked from commit 3c9ad9398fcdf5f49114fde978b7c837b7ebbc8d)