allow multi-attacks from monsters with more than one base attack

assume that the multi-attack is from multiple heads, and that the first
attack listed in their attack spec is the head attack
buff serpent of hell a bit

turn serpent of hell into a three headed dragon, each of which can bite
or use a breath weapon separately

unlike hydras, the heads can't be chopped off
also give serpent of hell summon dragon, because why not
buff the cocytus and dis variants a bit

swap out ozo's for glaciate and bump up the dis variant's ac and
physical attacks a bunch. still think the dis variant could use some
better breath attacks though.
few more tweaks
note "three-headed" in serpent of hell descriptions
better message for crystal spear breath
swap out flash freeze for glaciate

glaciate is a bit on the ridiculous side, and the messages are weird for
a breath weapon anyway