uinput device permissions

[Antoine Martin]

Follow up from #1611.

We want to be able to continue to use uinput when upgrading a server.

The patch attached almost kinda works, with some problems:

• requires "xprop" to find the device uuid... ugly
• the part of the patch that uses the device name to match the device cannot be used, as udev doesn't understand regex in the ATTRS match section... so we'll need to use something else (the device part of the USBID?)
• we chmod the device to 0666 initially so that the existing X11 server can add the new device, that's dangerous - we need to chmod / chown it to the user the server belongs to

[Antoine Martin]

work in progress patch

[Antoine Martin]

Done in r16777:

• we hijack the "version" attribute of the uinput device to pass the uid to our custom udev program which tells udev which "OWNER" value to use for the new device. That way, the permissions are correct as soon as the device is created.
• we also append a random string to the device name to ensure that the same user can run multiple sessions: each device will only be used by the server that knows its full name. We save the name as a root X11 property so we can re-create a new device when doing a server upgrade (or with --use-display).