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. This can result in situations where
mule PKs cannot attack anyone because they don't have enough HP, but they
can be attacked themselves. I intended this. In fact, liberally setting rooms to
piMinHpPK values of ~80 around mainland cities could seriously inhibit mule PKs
while not interfering with real combat between builts. (those changes not included here,
just something to think about)