lib/krb5_wrap: let smb_krb5_cc_get_lifetime() behave more like the heimdal krb5_cc_ge...

lib/krb5_wrap: let smb_krb5_cc_get_lifetime() behave more like the heimdal krb5_cc_get_lifetime

If the ccache doesn't have a intial TGT the shortest lifetime of
service tickets should be returned.

This is needed in order to work with special ccaches used for
things like S2U4Self/S4U2Proxy tickets or other things
where the caller only wants to pass a single service ticket.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>