Extend the interface to ether_input(): a NULL eh pointer means that
the mbuf contains the ethernet header (eh) as well, which ether_input()
will strip off as needed.

This permits the removal (in a backward compatible way) of the
header removal code which right now is replicated in all drivers,
sometimes in an inconsistent way. Also, because many functions
called after ether_input() require the eh in the mbuf, eventually
we can propagate the interface and handle outdated drivers just
in ether_input().

Individual driver changes to use the new interface will follow as
we have a chance to touch them.

NOTE THAT THIS CHANGE IS FULLY BACKWARD COMPATIBLE AND DOES NOT BREAK
BINARY COMPATIBILITY FOR DRIVERS.

MFC after: 3 days