Pets never displace player from stairs when entering a level

This removes the 1 in 10 chance for pets arriving on a level from
occupying the stairs and preventing the player from starting on the
stairs themselves.
Unhardcode Sokoban prizes

This moves the creation of Sokoban prizes out of fixup_special() code
and into the Sokoban level descriptions themselves.  This is done by
adding a new `prize` flag for `OBJECT` statements to the lev_comp
grammar which sends `SP_O_V_PRIZE` to the special level coder, in turn
setting the object's `oprize` flag (really `olocked` overloaded).

Object naming has been adjusted to allow for potentially any object to
be obfuscated out-of-sight and identified in-sight.

Constants and struct fields dealing with prizes that contain some
variation of the name "Sokoban" have been renamed to be more general.
Fix unobfuscated remembered old prize descriptions

Without this fix, the descriptions of prizes would leak if you picked up
a prize and then far-looked the locations of the other prizes.
Prevent destruction of prize items

This extends obj_resists() to include prizes, just in case they ever end
up in a situation where they would be destroyed before the player has
had a chance to pick one up.
Changelog for unobfuscated prize description fix
Move Sokoban prizes to Mines End

The prize at the end of Sokoban is now a sack with four random wands and
rings; the randomly-placed wands and rings on each Sokoban level have
been removed as a result.

This makes Sokoban less valuable and deeper Gnomish Mines more valuable,
making Sokoban now truly optional instead of optional-but-not-really.

This also guarantees a sack for characters unfortunate enough to not
find any bags in the entire dungeon.
Move Nymph level into Town branch; expand Town shops

The Nymph level is now guaranteed to appear between the Town surface and
filler levels.  The shops in town have been made bigger to compensate
for this.

Also, a random sink has been added to the Town surface to prevent cursed
levitation gear from making the player stuck.
Fix Town surface entry message grammar

"The" removed from "You hear the sounds of civilization."
lev_comp: Fix weight of contents added to containers

This was causing the Sokoban sack with four rings and wands to weigh 15
at first, then correct itself to weigh 55 once looted.