Add XIOErrorHandler and make poll() robust against X11 disconnection

[jEsuSdA]

Summary

Adds an XIOErrorHandler and makes the poll() loop robust against X11 connection errors, fixing silent exits when the X server disconnects.

Problem

Currently, when the X11 connection is lost (e.g. screen lock, VT switch, suspend/resume, or X server crash), fastcompmgr exits silently without any indication of what happened. This makes debugging very difficult — users just see the compositor "disappear".

Additionally, the poll() loop does not handle errors gracefully:

• POLLHUP/POLLERR/POLLNVAL on the X11 connection fd are ignored
• EINTR from poll() is not handled, causing the loop to break on signals
• Other poll errors are silently ignored

Solution

XIOErrorHandler

• Added xioerror_handler() that prints a clear "X11 connection lost (XIOError)" message to stderr before Xlib exits
• Registered via XSetIOErrorHandler() in main() right after XSetErrorHandler()

poll() robustness

• Check ufd.revents for POLLHUP/POLLERR/POLLNVAL and log which condition triggered with a clear fatal message
• Handle EINTR from poll() correctly: continue the loop instead of breaking
• Other poll() errors are logged with perror()

Scope

• Only fastcompmgr.c is touched
• make clean && make produces zero warnings, zero errors

Impact

Stability/reliability improvement with no functional changes to the compositing logic. Makes diagnosing X11 disconnection issues much easier.