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>

shell-completion/zsh/_pulseaudio

@@ -285,6 +285,7 @@ _pactl_completion() {
                 'clients: list connected clients'
                 'samples: list samples'
                 'cards: list available cards'
+                'message-handlers: list available message-handlers'
             )
 
             if ((CURRENT == 2)); then