Fix up a couple of descriptions.
Remove now-unused stat_gain_potion parameter from mutate().
Replace an outdated comment.
Make #ifdefs for removed needle brands reflect reality.

Needles of confusion have _not_ been removed, sickness have been.
Remove removed items from vaults.
Fix Mantis 6884: Manually uncurse hides after creating them.

This will ensure that they will never be cursed or get visible
descriptions (e.g. "runed"), since cursed items can get such
descriptions.
Fix Mantis 6862: Reword cloud trap message.

This uses mumra's suggested wording.
Set dont_place to true when placing hides made from corpses.

Since the hide is manually placed where the corpse used to be, it
doesn't need to be randomly placed.

Needed according to SamB on the ##crawl-dev logs.
Add -Wmissing-declarations to our default CFWARN flags.

This should help us spot unintentionally global functions, old unused
functions, and .cc files where we forgot to #include the corresponding
header.
Correct missing #include "foo.h" from foo.cc

(Also had to fix a prototype in ctest.h)
Correct other missing #includes
Make Static Discharge and elec melee consistent with other sources of elec damage

Static discharge, AF_ELEC and SPWPN_ELECTROCUTION were all affected by
flight as well as rElec, but all in different ways (SD did halved self
damage, no damage to enemies; AF_ELEC did 2/3 damage;
SPWPN_ELECTROCUTION did no damage).

All of these now act consistently and are just affected by rElec.
Fix spacing
Add back a Xom message

Imitating Vehumet was intentional, changed it to be low weight though.
Remove Jiyva's "food jelly" mutation

Jiyva tends to keep you at or close to Engorged most of the time, and
just spawning a neutral jelly isn't a very notable effect.
Remove some vampire special-casing for the gain stat mutations

No longer necessary with the potions removed.
Three new Jiyva mutations: disarming tendrils, item detection with a twist and absorbing missiles.
Don't drop missiles on the floor if a jelly mutation already ate them
Don't select Stalker as a random background
Remove now-inaccurate comment.