add an interface for sound libraries

Groundwork for a more versatile interface for using
sound libraries. A lot of sound libraries work across
multiple platforms.

The current NetHack sound stuff is quite limited.

Binaries can have a variety of window ports linked into
them, and it makes sense to have something similar for
sound.

This tries to set things up in a more soundlib-centric way,
rather than inserting things in a platform-centric way.

It establishes a new top-level directory sound (akin to win
for the window interface routines, or "window-port") where
sound-related additions and sndprocs and support files can be
added and used across platforms.

The default interface is nosound and the 'nosound' interface
is in src/sounds.c

The interface for 'windsound', which contains the same minimal
USER_SOUNDS support using built-in routines that has been in the
windows port for a long time is added to
sound/windsound/windsound.c.

For now, the sound interface support for 'qtsound' has been added
to the existing Qt files win/Qt/qt_bind.h and win/Qt/qt_bind.cpp,
and a note has been placed in sound/qtsound/README.md to avoid
confusion.

New header file added: include/sndprocs.h.

src/decl.c

@@ -206,6 +206,9 @@ const struct instance_globals_a g_init_a = {
     UNDEFINED_VALUE, /* abort_looting */
     /* shk.c */
     FALSE, /* auto_credit */
+    /* sounds.c */
+    soundlib_unassigned, /* enum soundlib_ids active_soundlib */
+
     /* trap.c */
     { 0, 0, FALSE }, /* acid_ctx */
     TRUE, /* havestate*/
@@ -284,6 +287,8 @@ const struct instance_globals_c g_init_c = {
     FALSE, /* class_filter */
     /* questpgr.c */
     UNDEFINED_VALUES, /* cvt_buf */
+    /* sounds.c */
+    soundlib_nosound, /* chosen_soundlib */
     UNDEFINED_PTR, /* coder */
     /* uhitm.c */
     NON_PM, /* corpsenm_digested */