Code freeze

[Gerd Moellmann]

I'm see that still changes are made, which are not essential for the
release of 21.1.  Every change has the potential of breaking
something, and therefore delay 21.1.  So, please postpone changes
which aren't bug fixes on the head branch until after 21.1 has been
released.

If you want to make 21.1 come out sooner so that new features can be
added on the head again, please pick an item from emacs-pretest-bug
and try to work on that; I think all core developers are now
subscribed to that list.  Working on items from emacs-bug is also
welcome :-).