Allow binding mouse buttons

Instead of hardcoding mouse button actions, allow the user to
bind mouse buttons to extended commands.  For example the new
defaults are:

BIND=mouse1:therecmdmenu
BIND=mouse2:clicklook

Currently a bit rudimentary; the defaults should be OK, but
documentation is bit lacking, and in-game binding and option
saving are missing.

Allowed commands to bind are "nothing", "therecmdmenu", "clicklook",
and "mouseaction". Clicklook replaces the "clicklook" boolean option,
and mouseaction does what mouse 1 button used to do - a context sensitive
action.

include/extern.h

@@ -279,6 +279,7 @@ extern void rhack(char *);
 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 void dokeylist(void);
 extern coordxy xytod(coordxy, coordxy);
@@ -291,7 +292,7 @@ extern const char *directionname(int);
 extern int isok(coordxy, coordxy);
 extern int get_adjacent_loc(const char *, const char *, coordxy, coordxy,
                             coord *);
-extern const char *click_to_cmd(coordxy, coordxy, int);
+extern void click_to_cmd(coordxy, coordxy, int);
 extern char get_count(const char *, char, long, cmdcount_nht *, unsigned);
 #ifdef HANGUPHANDLING
 extern void hangup(int);