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/mklev.c

@@ -1385,6 +1385,7 @@ mktrap(int num, int mktrapflags, struct mkroom *croom, coord *tm)
                 if (lvl < 5)
                     kind = NO_TRAP;
                 break;
+            case ANTI_MAGIC:
             case LANDMINE:
                 if (lvl < 6)
                     kind = NO_TRAP;