util-lib/systemd-run: implement race-free PTY peer opening (#34953)

util-lib/systemd-run: implement race-free PTY peer opening (#34953)

This makes use of the new TIOCGPTPEER pty ioctl() for directly opening a
PTY peer, without going via path names. This is nice because it closes a
race around allocating and opening the peer. And also has the nice
benefit that if we acquired an fd originating from some other
namespace/container, we can directly derive the peer fd from it, without
having to reenter the namespace again.