Tunes up mine pedestal finale

This adds some dark wyrms and noise to the initial sinkage, as well as making the eye emit Disturbing Screams so they can find you more easily.

More to the point, the wyrms can now dig just fine thanks to a new critter tag, MF_BORES, which gives 15x the bash ability a critter which BORES would typically have.  If a wyrm somehow got to the surface, it would wreck any fortifications you had, because it's pretty good at tunneling through solid rock.

(Until the sun came up, at which point it'd be Screwed.)

Fixes #10058.

Known issue, no known workaround: you can dodge the whole thing by not looking at the pedestal after removing the eye.  Not sure how best to fix that, short of having a loop specifically checking fr the presence of an eye on the pedestal.
OK, as soon as I finish with the merge I'll fix that.  Thanks.