[crossfire] Command learn_spell does not work for some spells

[Andreas Kirschbaum]

The DM command learn_spell does not work for some spells: for example
"learn_spell ball lightning" does not work.

The reason is that three different archetypes with "name ball lightning"
and "type 101" (SPELL) exist:
- abil_ball_lightning,
- spell_ball_lightning,
- spelldirect_ball_lightning.

This makes get_spell_by_name() fail because it finds more than one
matching spell and therefore rejects the name as ambiguous.

To fix this, I'd suggest to make this function consider only archetypes
with names "spell_xxx". Any objections or other suggestions?