Allow overriding exit() on I/O errors

Traditionally X11 I/O errors were fatal as there wasn't much point in allowing a UI application to live on without a mean to show the UI.

This is however not as crystal clear with Wayland compositors, as they may be x11 clients too (in order to serve as a X11 compositing manager) and it would be in their best interest to survive the fact.

This MR implements a XSetIOErrorExitHandler() function, that would allow compositors to try precisely that. I'm however unclear this is the best method, or something even sneakier (like an envvar) would be preferred, hence the WIP and the RFC.