Add message for creatures hit by cancellation and zero out hero's Pw

This was inspired initially by an EvilHack feature that added messages,
but I wasn't happy with the messages ("Magical energies are absorbed
from [monster]" implies the hero has magic-sight which isn't really the
case) so I changed them to an ambiguous visual effect.

The other change, removing all magic energy from the player when they
get canceled, is to provide a counterpart to monsters no longer being
able to use their spells or special attacks once cancelled. This
shouldn't actually happen very often, as the only causes right now are
the hero zapping cancellation at themselves (which also cancels their
entire inventory) or getting hit by Magicbane.

At some point I might consider implementing a player-side cancellation
status like FIQHack has, but not right now.

Author: copperwater

doc/xnh-changelog-10.0.md

@@ -36,9 +36,12 @@ changes:
 - The spell of restore ability has the same effects, except for curing illness.
 - Monsters hit by a potion of restore ability get cured of any blindness,
   confusion, or stunning, instead of being healed to full HP.
+- If the hero is hit with cancellation, they lose all their magic energy.
 
 ### Interface changes
 
+- There is a visual effect associated with the monster (or the hero) being
+  hit with cancellation.
 
 ### Architectural changes
 

src/zap.c

@@ -3242,6 +3242,12 @@ cancel_monst(struct monst *mdef, struct obj *obj, boolean youattack,
 
     /* now handle special cases */
     if (youdefend) {
+        if (!Blind)
+            pline("Dull sparkles flicker around you.");
+        /* since there is not (yet) a Cancelled status for heroes, at least
+         * temporarily limit their ability to use magical attacks... */
+        u.uen = 0;
+
         if (Upolyd) { /* includes lycanthrope in creature form */
             /*
              * Return to normal form unless Unchanging.
@@ -3267,6 +3273,8 @@ cancel_monst(struct monst *mdef, struct obj *obj, boolean youattack,
             }
         }
     } else {
+        if (canseemon(mdef))
+            pline("Dull sparkles flicker around %s.", mon_nam(mdef));
         mdef->mcan = 1;
         /* force shapeshifter into its base form or mimic to unhide */
         normal_shape(mdef);