zmacs: support message-log-max for *Messages* trimming.

* lib/zmacs/zlisp/minibuf.zl (message-log-max): Declare a new
variable for the maximum number of messages to keep in the
*Messages* buffer.
* lib/zile/minibuf.lua (minibuf_echo): Only add messages if
`message-log-max` is non-nil, and if it is a number then delete
the oldest messages until no more than that number remain.
* NEWS: Update.

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