Don't assume that utilities such as systemctl or initctl is found in a certain
specific path. Instead look for them in $PATH. This applies to both the
Makefile and the NetworkManager dispatcher script.
Resolves#11.
Newer NetworkManager versions will run the dispatcher scripts when new
unknown interfaces show up, including 'clat'. That will cause a clatd
restart right after startup, and we'll never get to fully initialise. So
ensure the dispatcher script does nothing if the device in question is
our own 'clat' to prevent this.
