Using 'P' to put on armour or 'W' to wear a ring

resulted in "That is a silly thing to [put on | wear]".

But those two cases really aren't so "silly", so adjust
the messages to better explain why the game objected
to the action and point new players at the appropriate
command.

Also adds a cmdassist message for the case where
'R' or 'T' have no appropriate items to point
new players to the correct command. (That can be
turned off with !cmdassist, of course.)

Also adds a const to a recent shade patch by request.

src/uhitm.c

@@ -912,7 +912,7 @@ int thrown;
 	    tmp = 0;
 	    if (mdat == &mons[PM_SHADE]) {
 		if (!hittxt) {
-		    char *what = unconventional[0] ? unconventional : "attack";
+		    const char *what = unconventional[0] ? unconventional : "attack";
 		    Your("%s %s harmlessly through %s.",
 		    	what, vtense(what, "pass"),
 			mon_nam(mon));