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/pulsecore/message-handler.h

@@ -26,7 +26,7 @@
 typedef int (*pa_message_handler_cb_t)(
         const char *object_path,
         const char *message,
-        const char *message_parameters,
+        char *message_parameters,
         char **response,
         void *userdata);