fix #H6628 - secret doors display as wrong wall

A relatively recent change to make secret doors within horizontal walls
become horizontal doors after discovery was making some secret doors
that should have remained vertical become horizontal too.  While still
hidden, they got displayed as horizontal wall segments in the midst
of vertical walls.  Example was the "Catacombs" (minend-3) variant of
mines' end.  The hidden door on the east wall of the entry room was
shown as horizontal, while another one on the west wall of that same
room was correctly vertical.  This fix uses different criteria to
decide horizontal vs vertical, partly because I couldn't understand
how the previous code was supposed to work.

Hidden doors now seem to display as correctly oriented walls and once
discovered seem to become correctly oriented doors.  I tested by
checking quite a few special levels (and some regular ones)--but not
all--with '#terrain d'.  Plus some searching to unhide secret doors
while using a custom symbol set that displayed closed horizontal doors
(S_hcdoor) as '=' and vertical ones (S_vcdoor) as '"'.

doc/fixes36.1

@@ -552,6 +552,9 @@ fix mention_walls reporting secret doors as solid stone
 jumping over water unintentionally moved hero through that water, causing
 	drowning if not able to water walk or fly
 try again to fix achievement recording bug with mines and sokoban prizes
+the fix for secret doors on special levels always having vertical orientation
+	resulted in some--but not all--secret doors within vertical walls
+	being displayed as horizontal walls while still hidden
 
 
 Platform- and/or Interface-Specific Fixes