SSHD configuration option for max number of displays

[Dmitry Belyavskiy]

Hello,

I have created a pull request
https://github.com/openssh/openssh-portable/pull/617

This patch is quite straightforward. It allows to specify the maximum
number of displays available for sshd's X11 forwarding.

This patch has been used for a while in Red Hat /Fedora and didn't cause
any problems. This is a rebase of this patch to current master branch

-- 
Dmitry Belyavskiy