Allow saving bones on levels that are currently ineligible due to a branch entrance

[copperwater]

YANI #1492: Make it possible to leave bones on certain levels that have a branch entrance in them (mainly mine entrance, sokoban entrance, quest), by virtue of removing the branch entrance before saving it as bones. If branch upstairs are removed in this manner, replace it with a pile of rocks; if downstairs are removed in this manner, replace them with a pit.

Levels with branch entrances will still be incapable of loading bones - this doesn't cover taking a bones level and figuring out an appropriate place to insert a required branch entrance.