pam: Clear up PAM_CONST related warnings on Linux

pam: Clear up PAM_CONST related warnings on Linux

Commit 78d1f8d8 expanded the use of PAM_CONST and introduced many
new warnings on Linux where pam expects "const" arguments.

This clears up the warnings by doing the following:
- Cast "user" to char * when kalling ka* functions
- Change the signature of pam_afs_prompt and pam_afs_printf to use
PAM_CONST
- Use a separate non-const password pointer for pam_afs_prompt

Change-Id: I460e1d1ca763f0aea5edcdaa208b9d4b8299ded0
Reviewed-on: http://gerrit.openafs.org/4487
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>