fix #H6925 - being trapped vs Levitation/Flying

Make being trapped in/on/over floor block Levitation and Flying, the
way that being inside solid rock already does, and the way levitating
blocks flight.

Blocked levitation still provides enhanced carrying capacity since
magic is attempting to make the hero's body be bouyant.  I think that
that is appropriate but am not completely convinced.

One thing that almost certainly needs fixing is digging a hole when
trapped in the floor or tethered to a buried iron ball, where the
first part of digactualhole() releases the hero from being trapped.
If being released re-enables blocked levitation, the further stages
of digging might not make sense in some circumstances.

I recently realized that being held by a grabbing monster is similar
to being trapped so should also interfere with levitation and flying.
Nothing here attempts to address that.

Save files change, but in a compatible fashion unless trapped at the
time of saving.  If someone saves while trapped prior to this patch,
then applies it and restores, the game will behave as if the patch
wasn't in place--until escape from trap is achieved.  (Not verified.)

include/youprop.h

@@ -209,8 +209,12 @@
 #define ETeleport_control u.uprops[TELEPORT_CONTROL].extrinsic
 #define Teleport_control (HTeleport_control || ETeleport_control)
 
+/* HLevitation has I_SPECIAL set if levitating due to blessed potion
+   which allows player to use the '>' command to end levitation early */
 #define HLevitation u.uprops[LEVITATION].intrinsic
 #define ELevitation u.uprops[LEVITATION].extrinsic
+/* BLevitation has I_SPECIAL set if trapped in the floor,
+   FROMOUTSIDE set if inside solid rock (or in water on Plane of Water) */
 #define BLevitation u.uprops[LEVITATION].blocked
 #define Levitation ((HLevitation || ELevitation) && !BLevitation)
 /* Can't touch surface, can't go under water; overrides all others */
@@ -219,8 +223,11 @@
      && (HLevitation & ~(I_SPECIAL | TIMEOUT)) == 0L                   \
      && (ELevitation & ~W_ARTI) == 0L)
 
+/* Flying is overridden by Levitation */
 #define HFlying u.uprops[FLYING].intrinsic
 #define EFlying u.uprops[FLYING].extrinsic
+/* BFlying has I_SPECIAL set if levitating or trapped in the floor or both,
+   FROMOUTSIDE set if inside solid rock (or in water on Plane of Water) */
 #define BFlying u.uprops[FLYING].blocked
 #define Flying                                                      \
     ((HFlying || EFlying || (u.usteed && is_flyer(u.usteed->data))) \