Fix interaction with systemd (#274)

The library is using inconsistent names when calling `systemctl`
commands.

Let's say that the service is named `name`. The unit file will be
then named `name.service` - this is the popular pattern used also by
this library.

While for most common commands like `systemctl start` there is no
difference whether `name` or `name.service` will be used as the service
identifier, the `list-unit-files` command expects explicitly the unit
file name.

Currently just the name - without the `.service` suffix - is being used,
which causes the command to return no output, not match the name of the
service and finally respond the status of installed but inactive service
as `StatusUnknown` with `ErrNotInstalled` error. While `StatusStopped`
without any error is expected.

Considering `systemctl` behavior it's just easier to:

- have one method that constructs the `name.service` name,
- use it for all `systemctl` commands calls.

And this is the change that this commit is adding.

service_systemd_linux.go

@@ -69,7 +69,7 @@ func (s *systemd) Platform() string {
 
 func (s *systemd) configPath() (cp string, err error) {
 	if !s.isUserService() {
-		cp = "/etc/systemd/system/" + s.Config.Name + ".service"
+		cp = "/etc/systemd/system/" + s.unitName()
 		return
 	}
 	homeDir, err := os.UserHomeDir()
@@ -81,10 +81,14 @@ func (s *systemd) configPath() (cp string, err error) {
 	if err != nil {
 		return
 	}
-	cp = filepath.Join(systemdUserDir, s.Config.Name+".service")
+	cp = filepath.Join(systemdUserDir, s.unitName())
 	return
 }
 
+func (s *systemd) unitName() string {
+	return s.Config.Name + ".service"
+}
+
 func (s *systemd) getSystemdVersion() int64 {
 	_, out, err := runWithOutput("systemctl", "--version")
 	if err != nil {
@@ -230,7 +234,7 @@ func (s *systemd) Run() (err error) {
 }
 
 func (s *systemd) Status() (Status, error) {
-	exitCode, out, err := runWithOutput("systemctl", "is-active", s.Name)
+	exitCode, out, err := runWithOutput("systemctl", "is-active", s.unitName())
 	if exitCode == 0 && err != nil {
 		return StatusUnknown, err
 	}
@@ -240,7 +244,7 @@ func (s *systemd) Status() (Status, error) {
 		return StatusRunning, nil
 	case strings.HasPrefix(out, "inactive"):
 		// inactive can also mean its not installed, check unit files
-		exitCode, out, err := runWithOutput("systemctl", "list-unit-files", "-t", "service", s.Name)
+		exitCode, out, err := runWithOutput("systemctl", "list-unit-files", "-t", "service", s.unitName())
 		if exitCode == 0 && err != nil {
 			return StatusUnknown, err
 		}
@@ -279,7 +283,7 @@ func (s *systemd) run(action string, args ...string) error {
 }
 
 func (s *systemd) runAction(action string) error {
-	return s.run(action, s.Name+".service")
+	return s.run(action, s.unitName())
 }
 
 const systemdScript = `[Unit]