mimics as doors on rogue level

mimics as doors on rogue level

     From a bug report.5 years ago? and again today:
mimics appear on the rogue level even though they're lowercase letter
monsters.  <Someone> notes that during level generation, mimics pretending to
be closed doors are sometimes substituted for trapped actual doors.  That
should be avoided on the rogue level both because its doorways are always
empty (enforced after the fact in extralev.c code rather than in the door
making code) and because mimics aren't uppercase letter monsters (so
should only ever appear there if they've travelled from another level).