Add ether_input_chain2() which could be called by ethernet NIC drivers to

Add ether_input_chain2() which could be called by ethernet NIC drivers to
deliver packets into upper layer.  Unlike ether_input_chain(), this function
only locates the msgport for the input packets, rest of the functionalities
of ether_input_chain() are performed in the protocal threads, i.e. after
packets reaches target msgports.  This function and other related functions
are put under kernel option ETHER_INPUT2, which is _not_ enabled by default.
As of this commit, vlan(4) and ipflow don't work with ether_input_chain2().

em(4) is adapted to aware ETHER_INPUT2.

Reviewed-by: dillon@, aggelos@ (early version)
# Additional comments are added to the the reviewed patch.