Support more than one clatd at a time.

1. Change from a single PID to a map of interface names to PIDs.
2. Make clatd commands take an interface argument.
3. Give the clatd processes names that depend on their parent
   interface, e.g., clatd-rmnet0.

Bug: 12111730
Change-Id: Ia770952d6511f8598a3356be5911da4ffa01b46a