libvncserver: Add channel security handlers

Add another type of security handler that is meant to be used initially
to set up a secure channel. Regular security handlers would be
advertised and processed after any channel security have succeeded.

For example, this, together with the custom I/O functions allows a
LibVNCServer user to implement TLS in combination with VNCAuth. This is
done by adding a single channel security handler with the rfbTLS (18)
with a handler that initiates a TLS session, and when a TLS session is
initiated, the regular security handler list is sent.