all: refactor router code to use interface

So the code is more modular, easier to read/maintain.

internal/router/openwrt/procd.go

@@ -0,0 +1,24 @@
+package openwrt
+
+const openWrtScript = `#!/bin/sh /etc/rc.common
+USE_PROCD=1
+# After network starts
+START=21
+# Before network stops
+STOP=89
+cmd="{{.Path}}{{range .Arguments}} {{.|cmd}}{{end}}"
+name="{{.Name}}"
+pid_file="/var/run/${name}.pid"
+
+start_service() {
+    echo "Starting ${name}"
+    procd_open_instance
+    procd_set_param command ${cmd}
+    procd_set_param respawn             # respawn automatically if something died
+    procd_set_param stdout 1            # forward stdout of the command to logd
+    procd_set_param stderr 1            # same for stderr
+    procd_set_param pidfile ${pid_file} # write a pid file on instance start and remove it on stop
+    procd_close_instance
+    echo "${name} has been started"
+}
+`