core: add support for naming file descriptors passed using socket activation

This adds support for naming file descriptors passed using socket
activation. The names are passed in a new $LISTEN_FDNAMES= environment
variable, that matches the existign $LISTEN_FDS= one and contains a
colon-separated list of names.

This also adds support for naming fds submitted to the per-service fd
store using FDNAME= in the sd_notify() message.

This also adds a new FileDescriptorName= setting for socket unit files
to set the name for fds created by socket units.

This also adds a new call sd_listen_fds_with_names(), that is similar to
sd_listen_fds(), but also returns the names of the fds.

systemd-activate gained the new --fdname= switch to specify a name for
testing socket activation.

This is based on #1247 by Maciej Wereski.

Fixes #1247.

src/core/dbus-socket.c

@@ -84,6 +84,25 @@ static int property_get_listen(
         return sd_bus_message_close_container(reply);
 }
 
+
+static int property_get_fdname(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *reply,
+                void *userdata,
+                sd_bus_error *error) {
+
+        Socket *s = SOCKET(userdata);
+
+        assert(bus);
+        assert(reply);
+        assert(s);
+
+        return sd_bus_message_append(reply, "s", socket_fdname(s));
+}
+
 const sd_bus_vtable bus_socket_vtable[] = {
         SD_BUS_VTABLE_START(0),
         SD_BUS_PROPERTY("BindIPv6Only", "s", property_get_bind_ipv6_only, offsetof(Socket, bind_ipv6_only), SD_BUS_VTABLE_PROPERTY_CONST),
@@ -128,6 +147,7 @@ const sd_bus_vtable bus_socket_vtable[] = {
         SD_BUS_PROPERTY("Result", "s", property_get_result, offsetof(Socket, result), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("NConnections", "u", bus_property_get_unsigned, offsetof(Socket, n_connections), 0),
         SD_BUS_PROPERTY("NAccepted", "u", bus_property_get_unsigned, offsetof(Socket, n_accepted), 0),
+        SD_BUS_PROPERTY("FileDescriptorName", "s", property_get_fdname, 0, 0),
         BUS_EXEC_COMMAND_LIST_VTABLE("ExecStartPre", offsetof(Socket, exec_command[SOCKET_EXEC_START_PRE]), SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION),
         BUS_EXEC_COMMAND_LIST_VTABLE("ExecStartPost", offsetof(Socket, exec_command[SOCKET_EXEC_START_POST]), SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION),
         BUS_EXEC_COMMAND_LIST_VTABLE("ExecStopPre", offsetof(Socket, exec_command[SOCKET_EXEC_STOP_PRE]), SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION),