In p4verify.sh, fixed handling of unload and spec depots.
Unload depots were not being identified correctly, and so special flags
needed for unload-type depots were not applied.
For spec-type depots, 'p4 verify -qS' was run, which isn't necessary or
helpful since it is not possible to shelve files in the spec depot.
The fix included replacing parsing logic with optimized p4 commands that
don't require parsing.