make /dev/pf a clonable device.

make /dev/pf a clonable device.

this provides a 1:1 relationship of pfopen() calls to pfclose()
calls. in turn, this makes it a lot easier to track stuff allocated
by a process and then clean it up if that process goes away
unexpectedly. the unique dev_t provided by the cloning machinery
gives us a good identifier to track this state with too.

discussed with h2k22
ok sashan@
deraadt@ agrees this is a good time to put this in