hdf5_msb

[svn-r18262] Purpose: Fix bug in b-tree code

Description:
In certain cases, removal of an object in a v1  b-tree would cause the leftmost
key in the right neighbor to be overwritten.  While this did not pose a problem
for group b-trees, with chunked datasets it would overwrite the offset value
of the neighbor's leftmost child, causing corruption.  Reworked the code to
differentiate between b-trees whose children are fundamentally associated with
their left key and those who are associated with their right key.

Tested: jam, linew, amani (h5committest)