fix issue #872 - container-to-container #tip

Reported by k2:  tipping one container's contents directly into
another container allowed transferring a wand of cancellation (not
mentioned:  or a bag of holding or a bag of tricks) into a bag of
holding without blowing it up.

That's now fixed.  There are other issues that this doesn't touch:

I think it's odd that you can transfer stuff from one carried
container to another but not from a carried container to a floor
container nor from one floor container to another one at same spot.

I didn't test shop billing so an not sure what happens when #tip
blows up a bag of holding and there are some unpaid items involved.

Using #tip on horn of plenty treats it like a container, but doing
that when it's carried doesn't offer the chance to tip its contents
directly into a carried container.

Tipping a carried container does not require free hands or even
limbs (for playability) but tipping such into another container
should require at least one free hand.

Fixes #872

doc/fixes3-7-0.txt

@@ -1380,6 +1380,9 @@ command line processing interleaved with options file processing is complex:
 when the Astral level got created, any fake player character monsters that
 	were seen or sensed when created were reported as "<mon> suddenly
 	appears!" rather than be treated as part of level's initial population
+tipping contents of one container directly into another allowed transferring
+	wands of cancellation and bags of holding or tricks that were inside
+	a sack, box, or chest into a bag of holding without blowing it up
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support