Add ExtraFileDescriptor property to StartTransientUnit dbus API

This adds the ExtraFileDescriptor property to StartTransient dbus API
with format "a(hs)" - array of (file descriptor, name) pairs. The FD
will be passed to the unit via sd_notify like Socket and OpenFile.

systemctl show also shows ExtraFileDescriptorName for these transient
units. We only show the name passed to dbus as the FD numbers will
change once passed over the unix socket and are duplicated, so its
confusing to display the numbers.

We do not add this functionality for systemd-run or general systemd
service units as it is not useful for general systemd services.
Arguably, it could be useful for systemd-run in bash scripts but we
prefer to be cautious and not expose the API yet.

Fixes: #34396

src/systemctl/systemctl-show.c

@@ -2017,6 +2017,21 @@ static int print_property(const char *name, const char *expected_value, sd_bus_m
                         if (r < 0)
                                 return bus_log_parse_error(r);
 
+                        return 1;
+                } else if (streq(name, "ExtraFileDescriptorNames")) {
+                        _cleanup_strv_free_ char **extra_fd_names = NULL;
+                        _cleanup_free_ char *joined = NULL;
+
+                        r = sd_bus_message_read_strv(m, &extra_fd_names);
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        joined = strv_join(extra_fd_names, " ");
+                        if (!joined)
+                                return log_oom();
+
+                        bus_print_property_value(name, expected_value, flags, joined);
+
                         return 1;
                 }