Accessibility: showdamage option

Add a new boolean option showdamage, if on, outputs a message
like "[HP -2, 14 left]" - several variants have something similar,
but I chose the message based on how eSpeak said it, while keeping
it short.

include/extern.h

@@ -1109,6 +1109,7 @@ extern void end_running(boolean);
 extern void nomul(int);
 extern void unmul(const char *);
 extern int saving_grace(int);
+extern void showdamage(int);
 extern void losehp(int, const char *, schar) ;
 extern int weight_cap(void);
 extern int inv_weight(void);

include/flag.h

@@ -213,6 +213,7 @@ struct accessibility_data {
  * and probably warrant a structure of their own elsewhere some day.
  */
 struct instance_flags {
+    boolean showdamage;
     boolean debug_fuzzer;  /* fuzz testing */
     boolean defer_plname;  /* X11 hack: askname() might not set gp.plname */
     boolean herecmd_menu;  /* use menu when mouseclick on yourself */

include/optlist.h

@@ -620,6 +620,9 @@ static int optfn_##a(int, int, boolean, char *, char *);
     NHOPTB(selectsaved, Advanced, 0, opt_out, set_in_config,
            On, Yes, No, No, NoAlias, &iflags.wc2_selectsaved, Term_False,
            (char *)0)
+    NHOPTB(showdamage, Status, 0, opt_in, set_in_game,
+           Off, Yes, No, No, NoAlias, &iflags.showdamage, Term_False,
+           "show damage hero takes in message line")
     NHOPTB(showexp, Status, 0, opt_in, set_in_game,
            Off, Yes, No, No, NoAlias, &flags.showexp, Term_False,
            "show experience points in status line")