add RFB parser

add RFB parser

This commit adds support for the Remote Framebuffer Protocol (RFB) as
used, for example, by various VNC implementations. It targets the
official versions 3.3, 3.7 and 3.8 of the protocol and provides logging
for the RFB handshake communication for now. Logged events include
endpoint versions, details of the security (i.e. authentication)
exchange as well as metadata about the image transfer parameters.
Detection is enabled using keywords for:

 - rfb.name: Session name as sticky buffer
 - rfb.sectype: Security type, e.g. VNC-style challenge-response
 - rfb.secresult: Result of the security exchange, e.g. OK, FAIL, ...

The latter could be used, for example, to detect brute-force attempts
on open VNC servers, while the name could be used to map unwanted VNC
sessions to the desktop owners or machines.

We also ship example EVE-JSON output and keyword docs as part of the
Sphinx source for Suricata's RTD documentation.