Room-based Minimum PK hp

Added a new room value & check that prevents combat between players if
one of them doesn't have enough HPs, as determined by the room's value
for piMinHpPK. This replaces our current use of the clunky GUILD_PK_ONLY
flag that discourages newbies from guilding and also allows built
players to farm low-level zones without risk. By making the protection
HP dependent, we refine the protection to solely those it's intended
for.