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.