Following https://github.com/cryptocode/ghostty/commit/7aeadb06ee5c38c440ac86b975d713a8ccfa3e0b

Fixes #5359

The comments explain what's going on. Longer term we should adjust our
termio/exec to avoid the SIGPIPE but its still possible (i.e. that
thread crashes) to happen so we should be robust to it.

Multiple fixes to prevent file descriptor leaks:

- libxev eventfd now uses CLOEXEC
- linux: cgroup clone now uses CLOEXEC for the cgroup fd
- termio pipe uses pipe2 with CLOEXEC
- pty master always sets CLOEXEC because the child doesn't need it
- termio exec now closes pty slave fd after fork

There still appear to be some fd leaks happening. They seem related to
GTK, they aren't things we're accessig directly. I still want to
investigate them but this at least cleans up the major sources of fd
leakage.