Bring r16846 from revise_chunks branch back to trunk:
Fix broken (for how long?) H5_ASSIGN_OVERFLOW macro to actually detect
overflows during assignments, along with several errors it [now] detected.
Cleaned up a fix minor warnings and/or pieces of code also.
Tested on:
FreeBSD/32 6.3 (duty) in debug mode
(h5committest not needed - multi-platform test performed on branch)
