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/pulse/meson.build

@@ -19,6 +19,7 @@ libpulse_sources = [
   'mainloop-api.c',
   'mainloop-signal.c',
   'mainloop.c',
+  'message-params.c',
   'operation.c',
   'proplist.c',
   'rtclock.c',
@@ -50,6 +51,7 @@ libpulse_headers = [
   'mainloop-api.h',
   'mainloop-signal.h',
   'mainloop.h',
+  'message-params.h',
   'operation.h',
   'proplist.h',
   'pulseaudio.h',