context-sensitive dipping

Normally dipping gets the thing to dip first and what to dip it
into second and the item-action handling knows that.  I'm not sure
why that wasn't working as intended and I couldn't figure out how
to make it do that, so went another way:  this adds an internal
extended command that executes an alternate dip routine which gets
the potion to dip into first and the thing to dip into it second.

The #dip command should allow an 'm' prefix to skip fountains and
pools, similar to how eating accepts it to skip food on the floor.
But this doesn't implement that.

src/cmd.c

@@ -2554,7 +2554,8 @@ struct ext_func_tab extcmdlist[] = {
             do_run_southwest, MOVEMENTCMD | CMD_M_PREFIX, NULL },
 
     /* internal commands: only used by game core, not available for user */
-    { '\0',   "clicklook", NULL, doclicklook, INTERNALCMD, NULL },
+    { '\0', "clicklook", NULL, doclicklook, INTERNALCMD, NULL },
+    { '\0', "altdip", NULL, dip_into, INTERNALCMD, NULL },
     { '\0', (char *) 0, (char *) 0, donull, 0, (char *) 0 } /* sentinel */
 };