rxgen: Make input strings const

rxgen: Make input strings const

Modify the code generation routines so that string inputs to RPCs
are declared as (const char *) on the client side. This doesn't affect
callers as we can freely cast from (char *) to (const char *), but means
it is easier to write API wrappers that accept const arguments.

Change-Id: I4719d04f03bd76cbe7ee21ad7511f6f3b3d36163
Reviewed-on: http://gerrit.openafs.org/7556
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>