Add "user canceled" as extended command return value

Instead of returning ECMD_OK, the commands now return ECMD_CANCEL
when user declined to pick a direction or an object to act on.

Note that this can be ORed with ECMD_TIME, if the command still
took a turn.

For now this has no gameplay meaning.

src/read.c

@@ -356,7 +356,7 @@ doread(void)
 
     scroll = getobj("read", read_ok, GETOBJ_PROMPT);
     if (!scroll)
-        return ECMD_OK;
+        return ECMD_CANCEL;
     otyp = scroll->otyp;
 
     /* outrumor has its own blindness check */