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.

include/optlist.h

@@ -540,6 +540,9 @@ static int optfn_##a(int, int, boolean, char *, char *);
     NHOPTC(sortvanquished, Advanced, 0, opt_in, set_in_game,
                 Yes, Yes, No, Yes, NoAlias,
                 "preferred order when displaying vanquished monsters")
+    NHOPTC(soundlib, Advanced, WINTYPELEN, opt_in, set_gameview,
+                No, Yes, No, No, NoAlias,
+                "soundlib interface to use (if any)")
     NHOPTB(sparkle, Map, 0, opt_out, set_in_game,
                 On, Yes, No, No, NoAlias, &flags.sparkle)
     NHOPTB(splash_screen, Advanced, 0, opt_out, set_in_config,