Fix the notify file descriptor getting inherited into the process group placeholder subprocess.

The easiest way to do that was to execl() a separate binary for the
placeholder (thereby closing all close-on-exec file descriptors, even
those we don't control, e.g. X11's), also making "ps" more descriptive
about what this process is for, and making sure any e.g. internal mutexes
by libraries we use are freed.

Fixes "systemd does not suspend laptop on closing the lid".