Tag forwarded X11 connection as remote

[Michal Srb]

Hi,

When X11 connection is forwarded using ssh, the ssh client typically connects 
to the local X server using unix socket (often it is the only option because X 
servers no longer listens on TCP by default). X clients connected remotely 
over ssh then seem like if they are local to the X server. Because of that it 
will attempt to use extensions that are meant for local clients only (SHM, 
DRI*, etc). In most cases the client or server can detect failure and fallback 
to a method that works remotely, but this does not always work:

https://bugzilla.opensuse.org/show_bug.cgi?id=1022727
(comments 24-26)

The ideal solution would be if ssh marked the connection as remote.
X protocol supports that since 2011:
https://cgit.freedesktop.org/xorg/xserver/commit/?
id=e2c7d70e5ddb8b17676a13ceebfbb87d14d63243

SSH only needs to change the first byte sent from X client to server to mark 
it as remote. SSH already modifies the whole first message (replaces 
authorization data), so changing the first byte is easy addition.

I have attached patch that implements it. Please check it and consider adding 
it or something similar to openssh.

Best regards,
Michal Srb
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