pactl: Implement list message-handlers

For better readability, "pactl list message-handlers" is introduced which
prints a formatted output of "pactl send-message /core list-handlers".

The patch also adds the functions pa_message_params_read_raw() and
pa_message_params_read_string() for easy parsing of the message response
string. Because the functions need to modify the parameter string,
the message handler and the pa_context_string_callback function now
receive a char* instead of a const char* as parameter argument.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/51>

src/map-file

@@ -229,6 +229,8 @@ pa_mainloop_quit;
 pa_mainloop_run;
 pa_mainloop_set_poll_func;
 pa_mainloop_wakeup;
+pa_message_params_read_raw;
+pa_message_params_read_string;
 pa_msleep;
 pa_thread_make_realtime;
 pa_operation_cancel;