add a master off/on switch for sounds

sounds can be set in the config file or on the fly with the Options menu.

This also adds a mechanism for specifying a terminology preference
for a boolean option in the options menu.

The choices are: Term_False, Term_Off, Term_Disabled

Term_False, the default, will use the terms "false" and "true" in the
Options menu.
Term_Off will use the terms "off" and "on" in the Options menu.
Term_Disabled will use the terms "disabled" and "enabled" in the Options
menu.

I didn't review any of the existing options to see if one of the new
alternative terms might be a better fit. They were all left at the default.

include/flag.h

@@ -265,6 +265,7 @@ struct instance_flags {
     boolean perm_invent;      /* keep full inventories up until dismissed */
     boolean renameallowed;    /* can change hero name during role selection */
     boolean renameinprogress; /* we are changing hero name */
+    boolean sounds;           /* master on/off switch for using soundlib */
     boolean status_updates;   /* allow updates to bottom status lines;
                                * disable to avoid excessive noise when using
                                * a screen reader (use ^X to review status) */