Make fire and frost breathe a PC skill with skill gains

The cost of fire and frost breathe are already ignored when it enters call_magic as CAST_BREATH not CAST_SPELL, so I've set them to 0 in spells.xml to avoid confusing characters when the skill appears on the spell sheet.

I've moved the spell number under 500 to allow for PCs to store a skill for it and have it handled normally by magical protection, INT increases for mages in mag_damage, and so forth. Previously it was above MAX_SPELLS and got ignored.

Finally, the breathe skill will be set to the same skill level as fire breathing if it isn't already. This lets the character gain skill in fire breathing since bringing the breath in via call_magic had been avoiding cast_magic which is normally what lets you gain skill.