sdp_upgrade.sh: Add ownership checks HxDepots/sdp, HxDepots/p4/common.
Ownership of files under /hxdepots/sdp and /hxdepots/p4/common
by any user other than the defined OSUSER (typically 'perforce')
will not be detected during a dry run, but will cause an upgrade
to abort safely and harmlessly in early-phase processing.
Even better behavior would be to detect the issue of incorrect file
ownership in the dry run.
Verify ownership of all files under /hxdepots/sdp and
/hxdepots/p4/common, the two directories modified by sdp_upgrade.sh,
and make sure all of the correct OSUSER ownership. If not, abort
with an appropriate error message.