Don't cheat Ash-skill-boosted chars out of their crosstraining bonus (#8901).

This function along with player::skill() badly need refactoring, but the
rounding situation is delicate enough (see 8b15fbdb4) that I'm not going
to refactor it right before 0.15.

(cherry picked from commit 6fc725f290d3fe117e168374fbdb0f29f7323411)
Don't let Fedhas worshippers crash at will.

To reproduce: use Growth, enter a number one greater than the number
of available adjacent spaces, then press enter.

Also, make an unreachable return use the correct type.

(cherry picked from commit 9fbdb5d7026b97ad6d968d4633fbba5eecc3fef2)
Don't displace Fedhas-walkable plants when taking stairs (#5117)

This could also be changed for submerged monsters (trapdoor spiders and
air elementals), but that is less noticeable.

(cherry picked from commit e697c9fcbaeccb31780277d8a472f23487a2e11c)
Add you.corrosion() to clua.

(cherry picked from commit 08341021b81f13b668f684beb63b17c6fa024c01)
Fix Song of Slaying description (Internet Kraken)

It claimed to only enhance melee; it also helps with ranged
combat!

Also tweak the following sentence for style + wordcount reasons.

(cherry picked from commit 8f376c1b5a09c1bcee708f2fce3cf5e51b65eeb1)