UniterAPI: Added AllMachinePorts; AssignedMachine; Fixed tag types

Changed a few things in the uniter API:
 - All client-side tags are now concrete tag kinds, rather than
   strings or generic names.Tag.
 - Removed GetOwnerTag server-side method and and replaced it with
   ServiceOwner, which does proper authorization checks and is a
   bulk call, fixing http://pad.lv/1270795 as a side-effect.
 - Added AssignedMachine method to return the tag of a unit's
   assigned machine.
 - Added AllMachinePorts method to return all opened port
   ranges (on all networks).
 - Added a Machine client-side proxy object, allowing Life
   to be called on machine tags.

These changes will be used in a follow-up that will make
the open-port and close-port hook tools sandboxed until
a hook is committed, while also ensuring each open-port
or close-port invocation also checks for conflicts.