Split ifnet serialize step 1/many: Add if_{serialize,deserialize,tryserialize}()

Split ifnet serialize step 1/many: Add if_{serialize,deserialize,tryserialize}()
function pointers to ifnet.

These three function pointers accept ifnet struct and ifnet_serialize
enumeration.

The ifnet_serialize enumeration indicates the serialization type:
IFNET_SERIALIZE_ALL:
    All of the serializers should be held.  Except for if_start and if_input,
    this enumeration must be used when call ifnet function pointers.
IFNET_SERIALIZE_TX:
    Only transmit serializer should be held.  This enumeration could be used
    when calling ifnet.if_start.
IFNET_SERIALIZE_RX:
    Only receive serializer should be held.  This enumeration could be used
    when calling ifnet.if_input.

If the NIC driver does not set these three function pointer, then if_attach()
will set them to the default ones: only one serializer (if_serializer) is used
and ifnet_serialize parameter is ignored.

Following several inline functions are added which are sheer wrappers of the
three ifnet serialize function pointers:
ifnet_serialize_{all,tx,rx}()
ifnet_deserialize_{all,tx,rx}()
ifnet_tryserialize_{all,tx,rx}()

All of the protocol layers and most of the pseudo drivers are converted.

Discussed-with: dillon@