pam: Password is const in setcred

pam: Password is const in setcred

afs_setcred.c gets the "password" pointer from pam_get_data, which
always gives a const pointer (unlike pam_get_item used in afs_auth.c
&c, which sometimes gives a const or not-const pointer, depending on
the PAM implementation).

So, declare password const, to get better type checking.

Change-Id: Ic34ffa54bf0bcc19c8ed3cddc9ee1384ee2dd8f0
Reviewed-on: http://gerrit.openafs.org/4553
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>