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/message-params.h

@@ -0,0 +1,42 @@
+#ifndef foomessagehelperhfoo
+#define foomessagehelperhfoo
+
+/***
+  This file is part of PulseAudio.
+
+  PulseAudio is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License as
+  published by the Free Software Foundation; either version 2.1 of the
+  License, or (at your option) any later version.
+
+  PulseAudio is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <stddef.h>
+#include <stdbool.h>
+#include <inttypes.h>
+
+#include <pulse/cdecl.h>
+#include <pulse/version.h>
+
+/** \file
+ * Utility functions for reading and writing message parameters */
+
+PA_C_DECL_BEGIN
+
+/** Read raw data from a parameter list. Used to split a message parameter
+ * string into list elements  \since 15.0 */
+int pa_message_params_read_raw(char *c, char **result, void **state);
+
+/** Read a string from a parameter list. \since 15.0 */
+int pa_message_params_read_string(char *c, const char **result, void **state);
+
+PA_C_DECL_END
+
+#endif