new symbol set: "plain"

new symbol set: "plain"

This set is the same as the default ascii symbols except that corner
walls are represented with '+' instead of '-' or '|' so that wall
joins are clearer.  The baalz level looks a little better this way,
although not a lot.  Unfortunately, most levels look a bit cluttered
with this, so I imagine it won't get a lot of use.  At least it
serves as an example of being able to use "'c'" instead of "\123".

Originally I specified every terrain symbol explicitly, which was
how I noticed that S_darkroom and S_vibrating_square weren't being
handled.  This has cut it down to just the wall symbols, serving as
explicit example of accepting default symbols for unspecified ones.