fix github issue 361 to make user_sounds useful even if MSGTYPE is hidden

fixes #361

Also, experminental introduction of vt_sounddata to enable tty to pass
a sound file index to the terminal side of things where perhaps someone
can add code to something like hterm to take the information relayed by
NetHack to trigger user_sounds locally even if playing on a server.

Compile time option TTY_SOUND_ESCCODES required to build that support in.

It should be independent of TTY_TILE_ESCCODES.

doc/Guidebook.tex

@@ -45,7 +45,7 @@
 %.au
 \author{Original version - Eric S. Raymond\\
 (Edited and expanded for 3.7 by Mike Stephenson and others)}
-\date{June 20, 2020}
+\date{July 2, 2020}
 
 \maketitle
 
@@ -5110,6 +5110,7 @@ Each SOUND entry is broken down into the following parts:
 {\tt pattern   } --- the pattern to match;\\
 {\tt sound file} --- the sound file to play;\\
 {\tt volume    } --- the volume to be set while playing the sound file.
+{\tt sound index } --- optional - the index corresponding to a sound file.
 %.ei
 %.ed
 \elist