Fix taking (and rezzing) of coalesced objects in the non-root subregions of megaregions.

This fixes the combined bounding box location for regions bigger than 256x256.
It also fixes the position on taking coalesced objects in the non-root regions, where position checks are properly done on rez instead.
It also fixes the megaregion land channel to return null if the land does not exist, which should probably also be done for the ordinary land channels rather than returning a dummy region.
Inspiration from Garmin's commit in http://opensimulator.org/mantis/view.php?id=6595.  Thanks!