Try all host keys by default in vfy_increds

Try all host keys by default in vfy_increds

Factor out the core code of krb5_verify_init_creds into a helper, add
new helper functions to retrieve the list of unique host principals
from a keytab, and make krb5_verify_init_creds drive the helper once
per host principal.

Augment the test harness and test cases to better test the new
behavior.  Add a k5test method to retrieve an NFS principal for the
test realm for the sake of the new test cases.

ticket: 7125

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25845 dc483132-0cff-0310-8789-dd5450dbe970