breaking migrated objects

breaking migrated objects
Allow migrated objects to break on arrival.  Added code to obj_delivery to
cause this, along with a flag to keep breakage from occurring.  The new
flag isn't used yet, because all the current object migration involve
objects that were moving/dropping.  To help make this change, rloco now
returns whether the object was placed or not, so caller can know if an obj
pointer is still valid or not.

Making the breakage messages for MIGR_NEAR_PLAYER objects show up after the
new level is displayed required some effort (rather than while the old level
was still displayed, which was confusing), due to the needs of goto_level.
- obj_delivery now has 2 passes, one for before player arrives, another after,
allowing the two cases to be treated differently
- goto_level calls obj_delivery twice (run_timers is not called twice,
since the run required before the level is displayed will have already run
any timers on migrating object)
- kill_genocided_monsters now kills eggs on the migrating_objs list too