Remove a pointless vault.

All it does is annoying and corroding people who:
1. happened to use mmap just on that particular level
2. are aware enough to notice
3. have tele+cTele
4. are unspoiled and don't know this particular trap
Place exactly one downward/upward stair between D:14 and D:15.

If we want a clear separation between the two, it helps to have a single
point of entry/exit.

A lot of the special casing would be better handled by having lower D be
a separate branch.
Four small vaults to place around the D:14 stair.

The idea I have here is to make them representative of what to expect
later in D.

More submissions to this effect are welcome.
Check the right spot for solidity.
Fix fake_pty not passing return codes correctly.
Show HOSTCC of fake_pty nicer.
Exclude certain uniques from Swamp/Snake/Shoals/Spider ends.

This addressed precisely which could only appear on the last level:
Boris, Frederick, Jory, Margery, Mennas, Sojobo, Xtahua.
Enable some other uniques on Swamp/etc branch ends.

These are all those which could spawn on Snake/etc:4 already:
Kirke, Louise, Nessos, Nikola, Norris, Roxanne, Rupert, Snorg.
Fix a small problem with a giant problem.

Oops.
Fix a slightly larger problem with a giant problem (|amethyst).
Prevent copies of spectral weapons

Destroy spectral weapon when leaving a level and it did not follow.
Don't give Yred piety for desecrating holy remains

You already get piety from the kill itself, and it leads to degenerate
behaviour (casting Sublimation of Blood on every single holy chunk for
maximum piety gain, etc).
Also require a rune to go from D:15 to D:14.

In case you somehow ended up there (for example early portal to Hell).
Allow D:14 shafts, make them known; try to always place one.

This is supposed to give players a one-way chance to bypass the rune lock.
When doing so, they have to procure a rune from Vaults or Hells or so in
order to be able to go back up to D:14.
Guarantee D:14 shaft vault.

The trick is to use DEPTH: D:14 and CHANCE: 100% instead of PLACE: D:14. (Grunt)
Rename the "staircase back to the Crypt/Forest".

It could refer to its exit dynamically, but that's not worth the effort.
Having different features for branch exits in the first place is dubious
to me, having them vary their names is worse.
Repair the "no OODs in hells" rule.

Not sure if it's still needed, it didn't work since 0.11.
Store the entry level for every branch (rather than just depth).

This allows moving branches around without breaking save compat or at least
serious hacks.

The portal stack can be probably dropped now: two copies of the same level
can't exist anyway.
A desc for monster frenzy.
Actually change thrashing horror's escape spell to Frenzy.

Also, tighten checks on whether it's allowed to fire.