control/controlclient, version/distro, wgengine: recognize OpenWrt

And help out with missing packages.

Thanks to @willangley for tips.

Updates #724

version/distro/distro.go

@@ -16,6 +16,7 @@ const (
 	Debian   = Distro("debian")
 	Arch     = Distro("arch")
 	Synology = Distro("synology")
+	OpenWrt  = Distro("openwrt")
 )
 
 // Get returns the current distro, or the empty string if unknown.
@@ -36,5 +37,8 @@ func linuxDistro() Distro {
 	if _, err := os.Stat("/etc/arch-release"); err == nil {
 		return Arch
 	}
+	if _, err := os.Stat("/etc/openwrt_version"); err == nil {
+		return OpenWrt
+	}
 	return ""
 }