pam: clean up unused variables and prototyping

pam: clean up unused variables and prototyping

don't define variables on platforms we won't use them on
do prototype functions we call. basically, we compile with warnings
enabled now, so, fix everything so we *can*.

Change-Id: I749f27c227ac70c58ccc68f1548f8274f10e3587
Reviewed-on: http://gerrit.openafs.org/5154
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>