Add verify option to cred store

Add verify option to cred store

The verify option instructs acquire_cred_from to verify a credential
obtained via a password, using the default keytab or the keytab
provided via the "keytab" key.  The value is a principal name (in
string form) for a key in the selected keytab, or the empty string to
use any host key in the keytab.

[ghudson@mit.edu: fleshed out tests; adjusted verify params contracts;
rewrote commit message]

ticket: 8963 (new)