GSSAPI tests and fixes

Changes in production code:
* Fixed `AbstractGssapiMechanism.DEFAULT_MAX_BUFFER_SIZE` (3 octets)
* Fixed bugs in `networkOrderBytesToInt()`, `intToNetworkOrderBytes()` and `parsePreferredQop()`
* ChallengeResponseState should return `byte[0]` instead of `null`
* SecurityLayerAdvertiser: `gssContext.getWrapSizeLimit()` moved on the
   opposite side of communication
* getNegotiatedProperty now returns Strings
* Allowed disabling of mutual authentication by client props (if not auth-int/auth-conf)