mon interaction with terrain was postponed

Fix #576

Reported initially by @copperwater for polymorphed monsters:
"observe how the [polymorphed-monster] hovers placidly above the water for
several turns like Wile E. Coyote before it gets a move, realizes it's
above water, and drowns. Ditto for lava."

A comment in the GitHub issue thread by @Tomsod pointed out that a
revived corpse could do the same.

This should set things up for other terrain fallout if discovered or
implemented in the future.
   - consume an additional bit in enum mon_terrain_effects (hack.h)
   - include the additional bit in TERRAIN_FALLOUT_MASK (monst.h)
   - add detection to maybe_set_terrain_effects (mon.c)
   - add action to terrain_effects (mon.c)

Implemented by stealing some upper unused mstate bits to avoid
invalidating existing save and bones.

include/monst.h

@@ -55,17 +55,23 @@ enum m_ap_types {
     M_AP_MONSTER   = 3  /* a monster; mostly used for cloned Wizard */
 };
 
-#define MON_FLOOR          0x0000
-#define MON_OFFMAP         0x0001
-#define MON_DETACH         0x0002
-#define MON_MIGRATING      0x0004
-#define MON_LIMBO          0x0008
-#define MON_BUBBLEMOVE     0x0010
-#define MON_ENDGAME_FREE   0x0020
-#define MON_ENDGAME_MIGR   0x0040
-#define MON_OBLITERATE     0x0080
-#define MON_STILL_ARRIVING 0x0100
-#define MON_PARKED         0x0200
+#define MON_FLOOR          0x00000000
+#define MON_OFFMAP         0x00000001
+#define MON_DETACH         0x00000002
+#define MON_MIGRATING      0x00000004
+#define MON_LIMBO          0x00000008
+#define MON_BUBBLEMOVE     0x00000010
+#define MON_ENDGAME_FREE   0x00000020
+#define MON_ENDGAME_MIGR   0x00000040
+#define MON_OBLITERATE     0x00000080
+#define MON_STILL_ARRIVING 0x00000100
+#define MON_PARKED         0x00000200
+/* 0x10000000 through 0x40000000 match enum pending_terrain_effects   */
+/*                         0x10000000 */ /* nonflyer_vs_liquid  */
+/*                         0x20000000 */ /* candrown_vs_liquid */
+/*                         0x40000000 */
+#define TERRAIN_FALLOUT_MASK   0x70000000
+
 
 #define M_AP_TYPMASK  0x7
 #define M_AP_F_DKNOWN 0x8