Make restart IPC command send a reply once restart completed (!) (#3743)

This is achieved by retaining the IPC connection which is sending the restart
command across the restart.

This is the cleaner fix for https://github.com/i3/go-i3/issues/3

fixes #3565

include/commands_parser.h

@@ -22,6 +22,10 @@ struct CommandResultIR {
     /* The JSON generator to append a reply to (may be NULL). */
     yajl_gen json_gen;
 
+    /* The IPC client connection which sent this command (may be NULL, e.g. for
+       key bindings). */
+    ipc_client *client;
+
     /* The next state to transition to. Passed to the function so that we can
      * determine the next state as a result of a function call, like
      * cfg_criteria_pop_state() does. */
@@ -61,7 +65,7 @@ char *parse_string(const char **walk, bool as_word);
  *
  * Free the returned CommandResult with command_result_free().
  */
-CommandResult *parse_command(const char *input, yajl_gen gen);
+CommandResult *parse_command(const char *input, yajl_gen gen, ipc_client *client);
 
 /**
  * Frees a CommandResult