Show pf fragment reassembly counters.

Show pf fragment reassembly counters.

Framgent count and statistics are stored in struct pf_status.  From
there pfctl(8) and systat(1) collect and show them.  Note that pfctl
-s info needs the -v switch to show fragments.  As fragment reassembly
has its own mutex, also grab this in pf ipctl(2) and sysctl(2) code.

input claudio@; OK henning@