Make NM/systemd integration more robust

This fixed the following dependency loop that could occur when the
system was booting up:

1. NetworkManager configures a network interface and fires off the clatd
   dispatcher script.
2. The dispatcher scripts tries to restart `clatd.service`.
3. However, `clatd.service` cannot be (re)started at this point because
   its dependency `network-online.target` has not yet been reached.
4. Therefore, the `systemctl restart clatd.service` command in the
   dispatcher script blocks, waiting for `network-online.target` to be
   reached.
5. But that won't happen until the dispatcher scripts finishes...

Adding `--no-block` allows the dispatcher script to finish immediately
without waiting for the restart to complete (instead, systemd will do it
in the background once `network-online.target` has been reached).

Furthermore, since `clatd.service` may end up being restarted several
times during a boot process (especially on a system with many network
interfaces handled by NetworkManager), it is also at risk of
inadvertently triggering the restart rate-limiting feature in systemd,
so disable that while we're at it.

Closes #22. Thanks to @patrakov for the report!

scripts/clatd.networkmanager

@@ -27,7 +27,7 @@
 
 # systemd-based distros
 if which systemctl &> /dev/null; then
-  systemctl restart clatd.service
+  systemctl --no-block restart clatd.service
 fi
 
 # upstart-based distros

scripts/clatd.systemd

@@ -12,6 +12,7 @@
 Description=464XLAT CLAT daemon
 Documentation=man:clatd(8)
 After=network-online.target
+StartLimitIntervalSec=0
 
 [Service]
 Type=simple