c/r: use /proc/self/tid/children instead of pidfile

c/r: use /proc/self/tid/children instead of pidfile

All we really needed a unique temp file for was passing the pid. Since CRIU
opened this with O_EXCL | O_CREAT, this was "safe" (users could still
overwrite it afterwards, but the monitor would immediately die since the
only valid number in there was the init process).

In any case, we can just read /proc/self/tid/children, which lists the
child process.

Closes #1150

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>