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.

Change-Id: I81c7863797396eb146b78ffbb2586e4f3a1e854e
Reviewed-on: http://gerrit.openafs.org/4470
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>