Weaken some extremely early overly dangerous vaults

Weaken some extremely early overly dangerous vaults

These vaults are some of the top killers in 0.14/0.15 D, and it's mostly
because they have some kind of unreasonable threat for an early, static
vault; usually this produces spoiler problems and anti-autoexplore
tendencies. Lemuel's castle is excluded for being classic and more
importantly difficult to miss warnings and nature, aside from bad
teleports (which can kill easily regardless of context).

Weakened vaults:
 * kennysheep_big_flooded_room uses D:2 9s: killer bees, ogres,
   centaurs. D:2-5 get 0s instead of 9s.
 * hangedman_cross_stitches's D:5 9s included hill giants,
   unseen horrors, wasps. Pushed down to D:6 minimum.
 * forest_paths uses quite a few clumped essentially-0s for
   D:2 / D:3 due to the map's 1/2/3 defaulting to 0s when
   not getting a set from the lua function. D:1-3 get
   several monsters trimmed out.
 * vehumet_altar_wand_db: A weakling with a weak wand is still
   extremely dangerous for an overflow altar on D:2. Pushed to D:3
   minimum.
 * fedhas_bush_and_centaur_altar: Oddly positioned compared to the
   others, due to the heavy centaur nerf from the ranged overhaul.
   Still, an overflow altar with a centaur in unfavourable terrain
   is a bit much for D:3. Pushed down to D:4 minimum.