There were couple reports of doors being generated in a corner
of a room. This happened for randomly generated irregular rooms,
because the code that was deciding if a corridor starting or
ending location was good did not handle irregular rooms at all.
This changes the corridor code so it can now generate start and
end points properly for any valid position in irregular rooms,
instead of only on the edges. This means the corridor sometimes
meanders a bit more than before, because it tries to find
the end point away from the edge of the room rectangle.
Also added is sanity checking for the randomly generated rooms
and corridors level, testing for door placement and room connectivity.
And another fix for a rare special case where dig_corridor
created a zero-tile long corridor; the entrance door was placed,
but there was nothing behind it.
Fixes github #1269 and #1385
b2c071bc66