Make split room levels less boring

The room creation function was double-counting split room halves when
limiting the number of rooms made per level, sometimes leading to boring
levels with just four rooms horizontally aligned.

This also makes split room creation respect MAXNROFROOMS.
More accurate cumulative heavyshot and artifact double damage

2x heavyshot damage coupled with 2x artifact damage should give 3x
instead of 4x, but since these damage modifiers are applied at different
places in the code, we add 1 to the multiplier and halve it to prepare
for the artifact code to do its damage doubling.

The issue that this commit solves is that the halving rounds when the
result is odd.  Previously, the code randomly chose for the end result
to be 1 higher or lower.  This commit replaces it by adding the
rounded-off part instead.

This is meant to go with commit f371c70783 (heavyshot: damage multiplier
instead of multishot).
Only quest nemesis can steal your quest artifact

In particular, your role's quest artifact can no longer be stolen by the
Wizard of Yendor, which is one of the biggest reasons NetHack players
almost always leave it behind.

The quest artifact is a special object with unique abilities that is
uniquely obtained by characters of each role, so it seems like an
obvious way to differentiate roles.
Generate normal liches on the Castle level

... instead of higher-level liches.  Since difficulty in this
development branch is now based only on depth, the random 'L' monsters
could sometimes be master liches and arch-liches, with no way for
players to mitigate the risk.
Guarantee shield of reflection on Medusa's Island

The statue of Perseus will now always contain a shield of reflection.
If it wouldn't have been generated before, it will also be uncursed, so
e.g. Titan Medusa has a good chance of having an shield of reflection
that's uncursed.
Grant magic resistance to role carrying quest artifact

This applies only to the player and not to monsters, at least for now.

This gives players more incentive to carry their role's quest artifact,
and less reason to wish for the quest artifacts of other roles.