Add template unit file

This allows the user to start espeakup service from the command-line
interface setting a specific voice/language without needing to edit
the unit file, which could be complicated for blind or visually
impaired users that do not understand English enough to hear English
espeakup before setting it to the desired language.

Example usage:
  systemd start espeakup@pt

services/systemd/espeakup@.service.in

@@ -0,0 +1,18 @@
+[Unit]
+Description=Software speech output for Speakup
+Documentation=man:espeakup(8)
+Wants=modprobe@speakup_soft.service
+After=modprobe@speakup_soft.service sound.target
+Conflicts=espeakup.service
+
+[Service]
+Type=forking
+PIDFile=/run/espeakup.pid
+ExecStart=@bindir@/espeakup --default-voice=%I
+ExecReload=kill -HUP $MAINPID
+Restart=always
+Nice=-10
+OOMScoreAdjust=-900
+
+[Install]
+WantedBy=sound.target

services/systemd/meson.build

@@ -11,6 +11,12 @@ service_file = configure_file(
   configuration : unit_conf
 )
 
-install_data(service_file,
+user_service_file = configure_file(
+  input : 'espeakup@.service.in',
+  output : 'espeakup@.service',
+  configuration : unit_conf
+)
+
+install_data(service_file, user_service_file,
   install_dir : unitdir
 )