GSS: reuse existing Subject and GssCredentials
Merge pull request #201 from simkam/eap_krb_2

gssapi: Re-use existing Subject and GssCredentials

The current implementation of `MakeGSS.java` always calls JAAS directly. In managed environments like application servers an upper level layer can handle authentication and then call `Driver.connect`. PgJDBC may not have access to the raw GSS credentials, or may be requiring the user to unnecessarily repeat them when the upper layers already have this information.

Allow PgJDBC to query the `AccessControllerContext` for GSS credentials and, if found, use existing credentials. If no `AccessControllerContext` exists, proceed as before, acquiring credentials directly.