bus: do kdbus only if this is enabled on the configure switch

Since we want to retain the ability to break kernel ←→ userspace ABI
after the next release, let's not make use by default of kdbus, so that
people with future kernels will not suddenly break with current systemd
versions.

kdbus support is left in all builds but must now be explicitly requested
at runtime (for example via setting $DBUS_SESSION_BUS). Via a configure
switch the old behaviour can be restored. In fact, we change autogen.sh
to do this, so that git builds (which run autogen.sh) get kdbus by
default, but tarball builds (which ue the configure defaults) do not get
it, and hence this stays out of the distros by default.

src/core/manager.c

@@ -414,6 +414,7 @@ static int manager_setup_kdbus(Manager *m) {
 
         assert(m);
 
+#ifdef ENABLE_KDBUS
         if (m->kdbus_fd >= 0)
                 return 0;
 
@@ -428,6 +429,8 @@ static int manager_setup_kdbus(Manager *m) {
         }
 
         log_info("Successfully set up kdbus on %s", p);
+#endif
+
         return 0;
 }
 

src/libsystemd-bus/sd-bus.c

@@ -993,7 +993,11 @@ _public_ int sd_bus_open_system(sd_bus **ret) {
         if (e)
                 r = sd_bus_set_address(b, e);
         else
+#ifdef ENABLE_KDBUS
                 r = sd_bus_set_address(b, "kernel:path=/dev/kdbus/0-system/bus;unix:path=/run/dbus/system_bus_socket");
+#else
+                r = sd_bus_set_address(b, "unix:path=/run/dbus/system_bus_socket");
+#endif
 
         if (r < 0)
                 goto fail;
@@ -1035,13 +1039,22 @@ _public_ int sd_bus_open_user(sd_bus **ret) {
 
                         ee = bus_address_escape(e);
                         if (!ee) {
-                                r = -ENOENT;
+                                r = -ENOMEM;
                                 goto fail;
                         }
 
+#ifdef ENABLE_KDBUS
                         asprintf(&b->address, "kernel:path=/dev/kdbus/%lu-user/bus;unix:path=%s/bus", (unsigned long) getuid(), ee);
-                } else
+#else
+                        b->address = strjoin("unix:path=", ee, "/bus", NULL);
+#endif
+                } else {
+#ifdef ENABLE_KDBUS
                         asprintf(&b->address, "kernel:path=/dev/kdbus/%lu-user/bus", (unsigned long) getuid());
+#else
+                        return -ECONNREFUSED;
+#endif
+                }
 
                 if (!b->address) {
                         r = -ENOMEM;