Rework garbage collector for unix(4) sockets.

Rework garbage collector for unix(4) sockets.

This time unix(4) sockets garbage collector always destroys any socket
with positive "fp->f_count == unp->unp_msgcount" equation. This is wrong
because unix(4) sockets within SCM_RIGHTS message but closed on sender
side also have this equation positive. Such sockets are not in the loop,
and if garbage collector kill them before they are received, we get
kernel panic.

FreeBSD already has garbage collector reworked to fix this issue [1]. The
logic is pretty simple so import it to our garbage collector.

1. https://reviews.freebsd.org/D23142

ok bluhm@