Merge 257643: Document RB_FOREACH_SAFE() and RB_FOREACH_REVERSE_SAFE().
Merge r257686: Add required kernel option.
Merge r257846:
  Make TCP_KEEP* socket options readable. At least PostgreSQL wants
  to read the values.
Merge several fixlets from head:

r257619: Remove unused PFTM_UNTIL_PACKET const.
r257620: Code logic of handling PFTM_PURGE into pf_find_state().
r258475: Don't compare unsigned <= 0.
r258477: Fix off by ones when scanning source nodes hash.