[gssproxy] [PATCHES] Improve credentials handling for init_sec_context

[Simo Sorce]

RFC
(patches are untested!)

If a client tries to call init_sec_context directly without explicitly
acquiring credentials first, try to see if we can automatically acquire
credentials on our own.

This will work only in some cases, when impersonation is allowed or when
a client keytab is available for the matching uid.

Simo.