Prevent monster generation in the sokoban trap hallway

Makes Sokoban far less tedious when you don't have to worry about
monsters randomly popping up in the trap hallway while you're pushing
the boulder.

Adds a new exclusion zone for monster generation, and the goodpos
routine avoids the zones when GP_AVOID_MONPOS is used.

doc/fixes3-7-0.txt

@@ -1237,7 +1237,7 @@ when a werecreature in human form attacked hero, it could transform to critter
 status highlighting for hit points didn't work as intended for up or down HP
 	changes; 'up' rule was used for both, 'down' rule was ignored
 unhide an unseen water monster using a polymorph trap on land
-allow defining random-teleport exclusion zones in lua
+allow defining random-teleport or monster-generation exclusion zones in lua
 if Magicbane cancelled a shapeshifter, forcing it to 'unshift', subsequent
 	messages continued to refer to the shifted form
 a pet that was poison resistant but not stoning resistant would eat Medusa's
@@ -1473,6 +1473,7 @@ interactively setting a status highlight for hunger with 'O' and choosing
 	use C++ regex processing but not for tty+posixregex
 a pet with the hides-under attribute could "move reluctantly over" a cursed
 	object and then hide under it
+prevent monster generation in the sokoban trap hallway
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

doc/lua.adoc

@@ -531,9 +531,10 @@ Example:
 === exclusion
 
 Exclude an area of the map from being randomly chosen target when
-falling or teleporting into the level.  Multiple exclusions per level are allowed.
+falling or teleporting into the level, or creating a monster.
+Multiple exclusions per level are allowed.
 
-* type is one of "teleport", "teleport-up", or "teleport-down".
+* type is one of "teleport", "teleport-up", "teleport-down", or "monster-generation".
 
 Example: