XMLs: remove useless os_level -1 from outputs, this value is the default and ignored
API/ABI: Remove obj->os_level

Not really documented, likely unused.

** This commit breaks the ABI **
API/ABI: reorganize fields in struct hwloc_obj

Move symmetric_subtree near children info.
Move userdata to the end.

We broke the ABI in the previous commit already
by removing one field.

** This commit breaks the ABI **
doxy: document the ABI break in the FAQ about API upgrades