Fix a couple of glitches and add an enchancement. The monster
attributes structure left the 'hidden' field uninitialized unless user
specified "hidden". Mimics were being flagged with mon->mundetected
because they pass the is_hider() test but they 'hide' by taking on an
appearance rather than being unseen due to mundetected. hides_under()
monsters fail the is_hider() test, but can become mundetected if there
is at least one object present. Eels/other fish are neither is_hider()
nor hides_under() but can be mundetected at water locations. So alter
'hidden' handling to deal with these various circumstances.
Asking for 'hidden' for any type of creature will result in having its
location be highlighted if it can't be actively seen or detected. So
using '2000 ^G piranha' will fill up the Plane of Water "normally" but
'2000 ^G hidden piranha' will result in a ton of draw-glyph/delay/
draw-other-glyph/delay sequences and take a painfully long time. Moral
of the story: don't combine 'hidden' with a large count unless you
want to spend quite a while watching the level's fill pattern. Turning
off the 'sparkle' option will cut the flashing in half but still take
a long time. If you really need to fill a level with hidden creatures
and can't bear the flashing/highlighting, use blessed potion of monster
detection or #wizintrinsics to have extended detect. Then all created
monsters will be seen so none will trigger location highlighting.
If you create a 'stalker' or 'invisible stalker' or 'invisible <other-
mon>' its location won't be highlighted, but for 'hidden stalker' or
'hidden invisible stalker' or 'hidden invisible <other-mon>' it will
(provided you don't actually see it due to See_invisible or sensemon()).
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