Bug fix: Necronomicon

There is some wierd problem with the necronomicon when it is found on
a level rather than obtained via a wish, corpsenumber (aka spestudied,
same value, multiple names) is set to an apparently random number.
Fixing it in artifact.c the first time the book is read works around
the problem. Explicitly giving a default value in mkobj.c did not.
Opaque monster bugfixes

Squashes two bugs related to opaque monsters: remove monster still refered
to the giant turtle specifically instead of opaque monsters generally,
and polymorphing a monster failed to unblock the map location, resulting in
phantom sight obstructions.
Beholder gaze bugfix

Cancelation gaze did not require line of sight. Was getting
canceled through walls.
New tiles for beholder and gas spore

Made the beholder and gas spore tiles more difficult to tell apart.