Show mon vs mon attack messages only when target is visible

The visibility check in mattackm doesn't guarantee both the
attacker and defender can be seen by hero. Before giving
messages, check more strictly whether we could see the
message happening - either really seeing the monster or
sensing the monster by some other means, depending on
the message.  This should remove most of the "It" messages.

Also unhide mimics who get gazed by umber hulks.  We could
keep the mimic hidden and make the messages reflect the
hulk gazing at the thing the mimic is mimicing, but this
is much easier.  This fixes bz631 / H4500

src/eat.c

@@ -498,7 +498,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
     } else if (mdef == &youmonst) {
         Your("brain is eaten!");
     } else { /* monster against monster */
-        if (visflag)
+        if (visflag && canspotmon(mdef))
             pline("%s brain is eaten!", s_suffix(Monnam(mdef)));
     }
 
@@ -512,7 +512,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
         } else {
             /* no need to check for poly_when_stoned or Stone_resistance;
                mind flayers don't have those capabilities */
-            if (visflag)
+            if (visflag && canseemon(magr))
                 pline("%s turns to stone!", Monnam(magr));
             monstone(magr);
             if (magr->mhp > 0) {
@@ -597,7 +597,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
          * monster mind flayer is eating another monster's brain
          */
         if (mindless(pd)) {
-            if (visflag)
+            if (visflag && canspotmon(mdef))
                 pline("%s doesn't notice.", Monnam(mdef));
             return MM_MISS;
         } else if (is_rider(pd)) {
@@ -609,7 +609,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
         } else {
             *dmg_p += xtra_dmg;
             give_nutrit = TRUE;
-            if (*dmg_p >= mdef->mhp && visflag)
+            if (*dmg_p >= mdef->mhp && visflag && canspotmon(mdef))
                 pline("%s last thought fades away...",
                       s_suffix(Monnam(mdef)));
         }