MFC: r244604, r244638, r245014

MFC: r244604, r244638, r245014
It was reported via email that some sshds create kerberos
credential cache files with names other than /tmp/krb5cc_<uid>.
The gssd daemon does not know how to find these credential caches.
This patch implements a new option "-s" that does a search for
credential cache files, using roughly the same algorithm as the
gssd daemon for Linux uses. The gssd behaviour is only changed
if the new "-s" option is specified.

git-svn-id: svn://svn.freebsd.org/base/stable/9@245089 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f