zile: support *Messages* buffer.

* lib/zile/buffer.lua (with_current_buffer): New function.
Quickly make a temporary buffer change, for the duration of
one callback function.
* lib/zile/minibuf.lua (messages_bp): New *Messages* buffer.
(minibuf_echo): Add a copy of minibuf messages to *Messages*.
* lib/zmacs/main.lua (main): Adjust buffer count when deciding
how to display windows on startup, now that we have an additional
*Messages* buffer.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>