Check new XLib IOError handler stuff

[uli42]

When the NX connection is lost nxagent as an X client to the real X server must not stop but continue to work. For this reason we have a patched libX11 that serves that purpose.

Recently there was a patch for libX11 that serves a similar purpose. It adds a IO error handler API: https://gitlab.freedesktop.org/xorg/lib/libx11/-/merge_requests/15

So if we could use that we could drop lots of changes in libX11 making updates simpler. I doubt we could drop the whole libX11 because we'd still need to bring in libxtrans somehow.

[uli42]

A prerequisite for this to check is to incorporate these changes into lib(NX_)X11, which is done in #1037