rx: Make struct rx_connection private

rx: Make struct rx_connection private

Move the rx_connection structure into a private header file, so that
it is only visible from within the rx module. This allows us to use
types within the structure that are not visible to everywhere that
includes rx.h, as well as being a step towards a more stable ABI for
RX.

Add accessor functions for all of the connection members which are
currently used by external callers, and modify those accessors
which were implemented as macros to also be functions.

Change all external access to the connection structures to use these
new functions.

Change-Id: Ife67e63f37cb04273fbfc9079db3907bde78ab98
Reviewed-on: http://gerrit.openafs.org/6180
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>