systemctl: include unit ID in the property map

Otherwise we get a not very nice message when trying to display a
non-existent man page:

~# systemctl cat test.service
[Unit]
Description=Hello
[Service]
ExecStart=true
~# systemctl help test.service
Documentation for (null) not known.

src/systemctl/systemctl-show.c

@@ -1996,6 +1996,7 @@ static int show_one(
                 bool *ellipsized) {
 
         static const struct bus_properties_map property_map[] = {
+                { "Id",                             "s",               NULL,           offsetof(UnitStatusInfo, id)                                },
                 { "LoadState",                      "s",               NULL,           offsetof(UnitStatusInfo, load_state)                        },
                 { "ActiveState",                    "s",               NULL,           offsetof(UnitStatusInfo, active_state)                      },
                 { "FreezerState",                   "s",               NULL,           offsetof(UnitStatusInfo, freezer_state)                     },