systemd: [plugin] add support for upstream unit files

Some modules install their own unit files during make install,
instead of disabling that behavior we can track those unit files
by extending our plugin and query the user for input to enable or disable
the service.

How?

Create a $SCRIPT_DIRECTORY/systemd_units file with the format (one unit per line):

daemon.service: A short description here
daemon2.socket: A short description here

system/systemd/plugin.d/systemd.plugin

@@ -13,16 +13,19 @@
 
 plugin_systemd_configure()
 {
-  local SERVICES SERVICE_FLAGGED SERVICE SYSTEMD_SERVICES
+  local SERVICES SERVICE_FLAGGED SERVICE SYSTEMD_SERVICES DESC
   debug_msg "plugin_systemd_configure ($@)"
 
   SYSTEMD_SERVICES=$(get_module_config SYSTEMD_SERVICES)
 
   if [ -d $SCRIPT_DIRECTORY/systemd.d ]; then
     cd $SCRIPT_DIRECTORY/systemd.d
-
     SERVICES=$(ls -1)
+  elif [ -f $SCRIPT_DIRECTORY/systemd_units ]; then
+    SERVICES=$(grep ':' $SCRIPT_DIRECTORY/systemd_units | awk -F': ' '{ print $1 }')
+  fi
 
+  if [ -n "$SERVICES" ]; then
     for SERVICE in $SERVICES; do
       # don't ask for '@' services - these should always be installed but
       # never linked directly. Also ask for new or renamed services.
@@ -34,7 +37,12 @@ plugin_systemd_configure()
 	      fi
         continue
       fi
-      message "${MESSAGE_COLOR}$SERVICE: $(grep Description= $SERVICE | cut -d= -f2-)${DEFAULT_COLOR}"
+      if [ -f $SCRIPT_DIRECTORY/systemd.d/$SERVICE ]; then
+        DESC=$(grep Description= $SCRIPT_DIRECTORY/systemd.d/$SERVICE | cut -d= -f2-)
+      else
+        DESC=$(grep "^$SERVICE:" $SCRIPT_DIRECTORY/systemd_units | awk -F': ' '{ print $2 }')
+      fi
+      message "${MESSAGE_COLOR}$SERVICE: $DESC${DEFAULT_COLOR}"
       if query "Invoke $SERVICE via systemd automatically at boot ?"  y
       then
         SYSTEMD_SERVICES+=" $SERVICE"