Use sorflush() instead of direct unp_scan(..., unp_discard) to discard

Use sorflush() instead of direct unp_scan(..., unp_discard) to discard
dead unix(4) sockets.

The difference in direct unp_scan() and sorflush() is the mbuf(9) chain.
For the first case it is still linked to the `so_rcv', for the second it
is not. This is required to make `sb_mtx' mutex(9) the only `so_rcv'
sockbuf protection and remove socket re-locking from the most of
uipc_*send() paths. The unlinked mbuf(9) chain doesn't require any
protection, so this allows to perform sleeping unp_discard() lockless.

Also, the mbuf(9) chain of the discarded socket still contains addresses
of file descriptors and it is much safer to unlink it before FRELE()
them. This is the reason to commit this diff standalone.

ok bluhm