systemd: get executable path properly

was just evaluating os.Args[0], which incorrectly points to your current
directory if jfa-go was in your PATH (i.e. you ran `jfa-go` not
`/usr/bin/jfa-go`). Uses Go's os.Executable() now. Fixes #352.

main.go

@@ -787,7 +787,7 @@ func main() {
 			fmt.Printf(lm.FailedReading+"\n", SYSTEMD_SERVICE, err)
 			os.Exit(1)
 		}
-		absPath, err := filepath.Abs(os.Args[0])
+		absPath, err := os.Executable()
 		if err != nil {
 			absPath = os.Args[0]
 		}