Add option to make commands ask an inventory item via menu

Turning the boolean option force_invmenu makes all the commands
that ask for an inventory item pop up a menu instead of asking
a text query.  This should be much more friendlier to new
players, and is very useful for window ports on systems
with touch screens and no physical keyboard, such as cell phones.

include/flag.h

@@ -229,6 +229,7 @@ struct instance_flags {
     boolean cbreak;           /* in cbreak mode, rogue format */
     boolean deferred_X;       /* deferred entry into explore mode */
     boolean echo;             /* 1 to echo characters */
+    boolean force_invmenu;    /* always menu when handling inventory */
     /* FIXME: goldX belongs in flags, but putting it in iflags avoids
        breaking 3.6.[01] save files */
     boolean goldX;            /* for BUCX filtering, whether gold is X or U */