klog: refactor klog_prompter

klog: refactor klog_prompter

The ifdefs in klog_prompter were getting a bit confusing. Split out
some logic into a separate "is this prompt a password prompt"
function. As a result, we can build without KRB5_PROMPT_TYPE_PASSWORD
defined, which happens to be the case on hp_ux11i.

Change-Id: I1d5f794bfc33017f699478e367cde91a3e77d33c
Reviewed-on: http://gerrit.openafs.org/2353
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>