pf: reorganise fragment reassembly

[kp]

Summary

To avoid packet loss due to reuse of the 16 bit IPv4 fragment id,
we need suitable data structures. Organize the pf fragments with
two red-black trees. One is holding the address and protocol
information and the other has only the fragment id. This will allow
to drop fragemts for specific connections more aggressively.
from markus@; OK sashan@

Obtained from: OpenBSD, bluhm <[e-mail]>, 09228e5ff0
Sponsored by: Rubicon Communications, LLC ("Netgate")