Change key binds from array to linked list

Key bindings were stored as a fixed-size array, indexed by the input
character, pointing to the extended commands.  This changes that into
a linked list of an intermediary struct Cmd_bind, storing the input key
and the pointer to the command.

This is just code cleanup for future enhancements, and should have
no effect on gameplay.

include/decl.h

@@ -240,6 +240,7 @@ struct instance_globals_c {
     /* decl.c */
     char chosen_windowtype[WINTYPELEN];
     int cmd_key; /* parse() / rhack() */
+    struct Cmd_bind *cmd_bind;
     cmdcount_nht command_count;
     /* some objects need special handling during destruction or placement */
     struct obj *current_wand;  /* wand currently zapped/applied */

include/extern.h

@@ -370,6 +370,7 @@ extern void change_palette(void);
 
 /* ### cmd.c ### */
 
+extern void cmdbind_freeall(void);
 extern void set_move_cmd(int, int);
 extern int do_move_west(void);
 extern int do_move_northwest(void);
@@ -447,7 +448,7 @@ extern int doextlist(void);
 extern int extcmd_via_menu(void);
 extern int enter_explore_mode(void);
 extern boolean bind_mousebtn(int, const char *);
-extern boolean bind_key(uchar, const char *);
+extern boolean bind_key(uchar, const char *, boolean);
 extern void dokeylist(void);
 extern int xytodir(int, int);
 extern void dirtocoord(coord *, int);

include/func_tab.h

@@ -29,6 +29,14 @@
 #define ECM_EXACTMATCH 0x02 /* needs exact match of findstr */
 #define ECM_NO1CHARCMD 0x04 /* ignore commands like '?' and '#' */
 
+/* a key bound to ext_func_tab */
+struct Cmd_bind {
+    uchar key;
+    boolean userbind; /* added by user */
+    const struct ext_func_tab *cmd;
+    struct Cmd_bind *next;
+};
+
 struct ext_func_tab {
     uchar key;
     const char *ef_txt, *ef_desc;

include/hack.h

@@ -252,7 +252,7 @@ struct cmd {
     boolean swap_yz;       /* QWERTZ keyboards; use z to move NW, y to zap */
     const char *dirchars;      /* current movement/direction characters */
     const char *alphadirchars; /* same as dirchars if !numpad */
-    const struct ext_func_tab *commands[256]; /* indexed by input character */
+    struct Cmd_bind *cmdbinds;
     const struct ext_func_tab *mousebtn[NUM_MOUSE_BUTTONS];
     char spkeys[NUM_NHKF];
     char extcmd_char;      /* key that starts an extended command ('#') */