networkd,resolved: make use of watch_bind feature to connect to the bus

The changes both networkd and resolved to make use of the watch_bind
feature of sd-bus to connect to the system bus. This way, both daemons
can be started during early boot, and automatically and instantly
connect to the system bus as it becomes available.

This replaces prior code that used a time-based retry logic to connect
to the bus.

src/shared/bus-util.c

@@ -1598,3 +1598,54 @@ int bus_track_add_name_many(sd_bus_track *t, char **l) {
 
         return r;
 }
+
+int bus_open_system_watch_bind(sd_bus **ret) {
+        _cleanup_(sd_bus_unrefp) sd_bus *bus = NULL;
+        const char *e;
+        int r;
+
+        assert(ret);
+
+        /* Match like sd_bus_open_system(), but with the "watch_bind" feature and the Connected() signal turned on. */
+
+        r = sd_bus_new(&bus);
+        if (r < 0)
+                return r;
+
+        e = secure_getenv("DBUS_SYSTEM_BUS_ADDRESS");
+        if (!e)
+                e = DEFAULT_SYSTEM_BUS_ADDRESS;
+
+        r = sd_bus_set_address(bus, e);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_set_bus_client(bus, true);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_set_trusted(bus, true);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_negotiate_creds(bus, true, SD_BUS_CREDS_UID|SD_BUS_CREDS_EUID|SD_BUS_CREDS_EFFECTIVE_CAPS);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_set_watch_bind(bus, true);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_set_connected_signal(bus, true);
+        if (r < 0)
+                return r;
+
+        r = sd_bus_start(bus);
+        if (r < 0)
+                return r;
+
+        *ret = bus;
+        bus = NULL;
+
+        return 0;
+}

src/shared/bus-util.h

@@ -162,3 +162,5 @@ int bus_path_decode_unique(const char *path, const char *prefix, char **ret_send
 int bus_property_get_rlimit(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *error);
 
 int bus_track_add_name_many(sd_bus_track *t, char **l);
+
+int bus_open_system_watch_bind(sd_bus **ret);