rx service specific data

rx service specific data

Adds rx_GetServiceSpecific and rx_SetServiceSpecific to the rx_service interface,
conforming to the equivalent calls in the rx_connection interface.
For consistency, the implementation strategy is the same.  The intended
use is to more cleanly support server multiplexing within an RPC-based
test dispatch library.

Change-Id: I6a8968484efe2c858857008321996e67328bb75c
Reviewed-on: http://gerrit.openafs.org/2097
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>