Add ability for service to report the system managing the service (#147)

* Add ability for service to report the system managing the service

* Clarify that the output of SystemName should return the same value as Platform in most cases

* Rename SystemName to Platform to match the rest of the package

* be a little more clear in the Platform comment

service_systemd_linux.go

@@ -24,14 +24,16 @@ func isSystemd() bool {
 }
 
 type systemd struct {
-	i Interface
+	i        Interface
+	platform string
 	*Config
 }
 
-func newSystemdService(i Interface, c *Config) (Service, error) {
+func newSystemdService(i Interface, platform string, c *Config) (Service, error) {
 	s := &systemd{
-		i:      i,
-		Config: c,
+		i:        i,
+		platform: platform,
+		Config:   c,
 	}
 
 	return s, nil
@@ -44,6 +46,10 @@ func (s *systemd) String() string {
 	return s.Name
 }
 
+func (s *systemd) Platform() string {
+	return s.platform
+}
+
 // Systemd services should be supported, but are not currently.
 var errNoUserServiceSystemd = errors.New("User services are not supported on systemd.")