rx: Tidy up function scope in rx.c

rx: Tidy up function scope in rx.c

Rework headers to more clearly indicate which functions in rx.c
are considered part of the 'public' rx interface, and which are for
internal use only.

Move internal functions either to be static in rx.c
(if they are used only within that file), or defined in
rx_internal.h.

Remove entirely the unused function rxi_NatKeepAliveOn()

Change-Id: I878374dc137f4c7e204de4b2d9ca17fa1c5db628
Reviewed-on: http://gerrit.openafs.org/6998
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>