Split out handlers for SOCK_DGRAM unix(4) sockets from SOCK_STREAM and

Split out handlers for SOCK_DGRAM unix(4) sockets from SOCK_STREAM and
SOCK_SEQPACKET. Introduce `uipc_dgram_usrreqs' to store pointers for
dgram specific handlers.

The dgram pru_shutdown and pru_send handlers were splitted to
uipc_dgram_shutdown() and uipc_dgram_send(). The pru_accept, pru_rcvd
and pru_abort handlers are not required for dgram sockets.

The unp_disconnect() remains shared between all unix(4) sockets because
it called from common paths too.

Proposed by and ok guenther@