r9728: A *major* update to the credentials system, to incorporate the

r9728: A *major* update to the credentials system, to incorporate the
Kerberos CCACHE into the system.

This again allows the use of the system ccache when no username is
specified, and brings more code in common between gensec_krb5 and
gensec_gssapi.

It also has a side-effect that may (or may not) be expected: If there
is a ccache, even if it is not used (perhaps the remote server didn't
want kerberos), it will change the default username.

Andrew Bartlett
(This used to be commit 6202267f6ec1446d6bd11d1d37d05a977bc8d315)