pam: Use PAM_CONST more often

pam: Use PAM_CONST more often

Some callers of pam_get_item et al were just casting their argument to
a const void **. Some PAM implementations (Linux) want a const void**,
but others (Solaris) do not. Use the PAM_CONST symbol already defined
by autoconf to declare or cast the relevant variable const or not as
appropriate.

Reviewed-on: http://gerrit.openafs.org/4470
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 78d1f8d88334f711eaaf6555d3a962a504d3e80e)

Change-Id: I831fa52c238a6cf7ef211e8198815c4420ae7dce
Reviewed-on: http://gerrit.openafs.org/6291
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>