Implement shared-storage full backup/restore

Every available shared-storage volume is backed up, tagged with its
ordinal in the set of mounted shared volumes.  This is an approximation
of "internal + the external card".  This lets us restore things to the
same volume [or "equivalent" volume, in the case of a cross-model
restore] as they originated on.

Also fixed a bug in the handling of files/dirs with spaces in
their names.

Change-Id: I380019da8d0bb5b3699bd7c11eeff621a88e78c3