Use system properties to track the current transport

This change retools the transport selection mechanism a fair bit.  Transports
are now specified by name rather than by numeric ID, and the name of the
currently selected transport is stored in a persistent system property under the
name "persist.service.bkup.trans".

The name -> IBackupTransport translation is now handled by maintaining a map
from the names to the live IBackupTransport objects that correspond.  The Google
transport service observer now registers and unregisters the transport as the
service goes up and down.

The bmgr command has been expanded to include real transport interrogation and
selection by name, and some documentation has been written for it.