Make anti-magic fields drain more energy

... and make them actually deal damage based on the energy
it would've drained, if you have Antimagic.
Also prevent them appearing too early in the dungeon.

Allow drain energy attacks (and anti-magic traps) drain more
than your level of energy.

Make eating magical monsters such as wizards and shamans give
the same tiny buzz bonus as eating a newt.

src/trap.c

@@ -1974,15 +1974,17 @@ trapeffect_anti_magic(
     unsigned int trflags UNUSED)
 {
     if (mtmp == &g.youmonst) {
+        int drain = (u.uen > 1) ? (rnd(u.uen / 2) + 2) : 4;
+
         seetrap(trap);
         /* hero without magic resistance loses spell energy,
            hero with magic resistance takes damage instead;
            possibly non-intuitive but useful for play balance */
         if (!Antimagic) {
-            drain_en(rnd(u.ulevel) + 1);
+            drain_en(drain);
         } else {
             struct obj *otmp;
-            int dmgval2 = rnd(4), hp = Upolyd ? u.mh : u.uhp;
+            int dmgval2 = rnd(drain), hp = Upolyd ? u.mh : u.uhp;
 
             /* Half_XXX_damage has opposite its usual effect (approx)
                but isn't cumulative if hero has more than one */
@@ -4293,7 +4295,7 @@ drain_en(int n)
         You_feel("momentarily lethargic.");
     } else {
         /* throttle further loss a bit when there's not much left to lose */
-        if (n > u.uenmax || n > u.ulevel)
+        if (n > (u.uen + u.uenmax) / 3)
             n = rnd(n);
 
         You_feel("your magical energy drain away%c", (n > u.uen) ? '!' : '.');