refactor: add get_buffer_create and simplify accordingly.

* lib/zile/buffer.lua (get_buffer_create): New function. Return
an existing buffer, or create one if necessary.
* lib/zile/minibuf.lua (messages_bp): Remove.  Recreate the
"*Messages*" buffer on demand with get_buffer_create.
(trim_messages): New function.  Factored out of...
(minibuf_echo): ...here.
* lib/zmacs/main.lua (main): "*Messages*" is not created until
required, so revert to counting only "*scratch*" as an additional
buffer when deciding how to split windows at startup.

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