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.
We could also potentially create wide swaths of rooms (main gates, and
other low level areas) that are safe for newbies but that don't interfere with
combat between built players.

Notable consequences:

With the default for all rooms set to 30, this could simply replace the angel
system entirely.

If this replaces the angel system, we can make rooms where mules
are vulnerable by lowering the setting below 30. That might be appropriate for
boss rooms and high-end content.

Outlaws and murderers are not protected on the victim end.
Edit: Nor on the attacker end with the second commit. So for example, a 150hp
white player and a 35 hp red can fight each other in a zone with a min limit of 75.
But a 40 hp white player in that same zone is protected, and cannot attack the red.