Move PRU_ABORT request to (*pru_abort)().

Move PRU_ABORT request to (*pru_abort)().

We abort only the sockets which are linked to `so_q' or `so_q0' queues of
listening socket. Such sockets have no corresponding file descriptor and
are not accessed from userland, so PRU_ABORT used to destroy them on
listening socket destruction.

Currently all our sockets support PRU_ABORT request, but actually it
required only for tcp(4) and unix(4) sockets, so i should be optional.
However, they will be removed with separate diff, and this time  PRU_ABORT
requests were converted as is.

Also, the socket should be destroyed on PRU_ABORT request, but route and
key management sockets leave it alive. This was also converted as is,
because this wrong code never called.

ok bluhm@