Remove internet PCB mutex.

Remove internet PCB mutex.

All incpb locking has been converted to socket receive buffer mutex.
Per PCB mutex inp_mtx is not needed anymore.  Also delete PRU related
locking functions.  A flag PR_MPSOCKET indicates whether protocol
functions support parallel access with per socket rw-lock.
TCP is the only protocol that is not MP capable from the socket
layer and needs exclusive netlock.

OK mvs@