This adds RFB 3.8 support to vino. The only changes relative to 3.7 are two
tiny changes to authentication.

However, the patch still makes vino only advertise 3.7 support by default,
because 3.8 requires the server to send an error message if authentication
fails, but the error message has to be iso8859-1 encoded, meaning it can't
be fully localized, meaning that in most cases, the user will fare better
with a generic-but-localized client-side error message than with the
specific-but-unlocalized server-side error message. (Particularly since we
only support password auth, and there's only one error that can occur with
it...)

So what's the point of adding 3.8 support if we only advertise 3.7 support?

    1. At least one client (RealVNC 4.1.9) is broken and requires the server
       to use 3.8 even if it only advertises 3.7 (and so it will hang when
       connecting to vino if you *don't* use password auth because of the
       protocol changes).

    2. In the future, we may need/want 3.8 support for other reasons (as a
       stepping stone to 3.9?), and I've already written the code, so here
       it is.