MFC: Close a race between enumerating UNIX domain socket pcb structures via

MFC: Close a race between enumerating UNIX domain socket pcb structures via
sysctl and socket teardown.  Note that we engage in a bit of trickery to
preserve the ABI of 'struct unpcb' in 6.x.  We change the UMA zone to hold
a 'struct unpcb_wrapper' which holds a 6.x 'struct unpcb' followed by the
new reference count needed for handling the race.  We then cast 'struct
unpcb' pointers to 'struct unpcb_wrapper' pointers when we need to access
the reference count.

Submitted by:	ups (including the ABI trickery)