More monster tile weapon and shield offsets

Thus my summoner can fully evolve to its final form.
Merfolk tile offset adjustments
Remove a hardcoded branch depth (|amethyst).

Not that we are particularly likely to change Zot's depth.
Be a little less violent about invalid traps.

On second thought, crashing is a bit too violent, especially for people
just trying out writing a vault. There's no reason to interrupt someone's
play just because the vault designer made a mistake, either.
CLua: add you.best_skill().

Probably not very useful outside of the novelty glyph-changing rc,
admittedly.
Allow mon_glyph = <mon1>:<mon2>.
Deduplicate code.
Don't autotarget friendlies with lamp of fire (#8988)