src/pam warning fixes

src/pam warning fixes

The second parameter to pam_conv() should be a const pointer on
recent systems.  Make it so to eliminate a couple of warnings.
A configure test is added to deal with some systems where pam_conv()
might not be const.

Cast a few assignments to cell_ptr in afs_auth.c and afs_setcred.c
since the argv parameter is const.

Change-Id: I5757310c94a6f26ca7dab656edaa416d16e32e2a
Reviewed-on: http://gerrit.openafs.org/847
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>