Set l4 pointer properly in buffers that contain IP options.

Set l4 pointer properly in buffers that contain IP options.

The important part of this change is handling of IP options.  The
rest is intended to ensure that packet->l3 and packet->l4 never
point outside the valid range of a packet in the presence of a
minimum-length Ethernet frame.  Other code (particularly in forward.c)
assumes that if the IP protocol is TCP or UDP, then packet->l4 points
to a tcp_header or udp_header.  Whether this is a good idea is
debatable--it probably warrants clean-up, at least--but that what's
there now.