Somewhat re-factor the read/write locking mechanism associated with the packet
filtering mechanisms to use the new rwlock(9) locking API:

• Drop the variables stored in the phil_head structure which were specific to conditions and the home rolled read/write locking mechanism.
• Drop some includes which were used for condition variables
• Drop the inline functions, and convert them to macros. Also, move these macros into pfil.h
• Move pfil list locking macros intp phil.h as well
• Rename ph_busy_count to ph_nhooks. This variable will represent the number of IN/OUT hooks registered with the pfil head structure
• Define PFIL_HOOKED macro which evaluates to true if there are any hooks to be ran by pfil_run_hooks
• In the IP/IP6 stacks, change the ph_busy_count comparison to use the new PFIL_HOOKED macro.
• Drop optimization in pfil_run_hooks which checks to see if there are any hooks to be ran, and returns if not. This check is already performed by the IP stacks when they call:
  
  if (!PFIL_HOOKED(ph)) goto skip_hooks;

• Drop in assertion which makes sure that the number of hooks never drops below 0 for good measure. This in theory should never happen, and if it does than there are problems somewhere
• Drop special logic around PFIL_WAITOK because rw_wlock(9) does not sleep
• Drop variables which support home rolled read/write locking mechanism from the IPFW firewall chain structure.
• Swap out the read/write firewall chain lock internal to use the rwlock(9) API instead of our home rolled version
• Convert the inlined functions to macros

Reviewed by: mlaier, andre, glebius
Thanks to: jhb for the new locking API