More pam warning cleanups - test_pam.c

More pam warning cleanups - test_pam.c

Clear up a few more warnings in the pam code.  The changes here
are very similar to the ones in the previous patch and use
the same configure test:
- use PAM_CONST to conditionally declare pam_message as const
- cast a few arguments to putenv, which expects a non-const pointer

Change-Id: I6c98623c35f4453f34c1d48b8b7d6ff1bfbc1e0c
Reviewed-on: http://gerrit.openafs.org/1116
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>