tree-wide: do not print hint about -M if -M is already used

(Or when -H is used, since -H and -M are incompatible.)

Note that the slightly unusual form with separate boolean variables (hint_vars,
hint_addr) instead of e.g. a const char* variable to hold the message, because this
way we don't trigger the warning about non-literal format.

src/shared/bus-util.c

@@ -42,11 +42,14 @@ int bus_log_address_error(int r, BusTransport transport) {
                                       "Failed to set bus address: %m");
 }
 
-int bus_log_connect_error(int r) {
+int bus_log_connect_error(int r, BusTransport transport) {
+        bool hint_vars = transport == BUS_TRANSPORT_LOCAL && r == -ENOMEDIUM,
+             hint_addr = transport == BUS_TRANSPORT_LOCAL && ERRNO_IS_PRIVILEGE(r);
+
         return log_error_errno(r,
-                               r == -ENOMEDIUM       ? "Failed to connect to bus: $DBUS_SESSION_BUS_ADDRESS and $XDG_RUNTIME_DIR not defined (consider using --machine=<user>@.host --user to connect to bus of other user)" :
-                               ERRNO_IS_PRIVILEGE(r) ? "Failed to connect to bus: Operation not permitted (consider using --machine=<user>@.host --user to connect to bus of other user)" :
-                                                       "Failed to connect to bus: %m");
+                               r == hint_vars ? "Failed to connect to bus: $DBUS_SESSION_BUS_ADDRESS and $XDG_RUNTIME_DIR not defined (consider using --machine=<user>@.host --user to connect to bus of other user)" :
+                               r == hint_addr ? "Failed to connect to bus: Operation not permitted (consider using --machine=<user>@.host --user to connect to bus of other user)" :
+                                                "Failed to connect to bus: %m");
 }
 
 int bus_async_unregister_and_exit(sd_event *e, sd_bus *bus, const char *name) {

src/shared/bus-util.h

@@ -40,7 +40,7 @@ int bus_connect_transport(BusTransport transport, const char *host, bool user, s
 int bus_connect_transport_systemd(BusTransport transport, const char *host, bool user, sd_bus **bus);
 
 int bus_log_address_error(int r, BusTransport transport);
-int bus_log_connect_error(int r);
+int bus_log_connect_error(int r, BusTransport transport);
 
 #define bus_log_parse_error(r)                                  \
         log_error_errno(r, "Failed to parse bus message: %m")

src/shared/cgroup-show.c

@@ -447,7 +447,7 @@ int show_cgroup_get_path_and_warn(
 
                 r = bus_connect_transport_systemd(BUS_TRANSPORT_LOCAL, NULL, false, &bus);
                 if (r < 0)
-                        return bus_log_connect_error(r);
+                        return bus_log_connect_error(r, BUS_TRANSPORT_LOCAL);
 
                 r = show_cgroup_get_unit_path_and_warn(bus, unit, &root);
                 if (r < 0)