rx: Make the rx_call structure private

rx: Make the rx_call structure private

Hide the rx_call structure for public view. Provide accessors for
those elements which are currently accessed by applications.

Note that this change as it currently stands removes the visibility
of the last sent time, and sequence number information, from the
VolMonitor function.

Change-Id: Ib25ab5635126f893ae43acb684d92a78278d6ca6
Reviewed-on: http://gerrit.openafs.org/6181
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>