zmacs: use new minibuf_echo for non-error information in minibuf.

* lib/zile/minibuf (minibuf_echo): New function. Write message
to minibuf, or stdout in batch mode.
(minibuf_error): Adjust to write to stderr in batch mode.
(minibuf_refresh): remove batch mode code, and simplify.
* lib/zile/term_minibuf.lua (term_minibuf_write): Likewise.
Adjust `minibuf_write` calls to use `minibuf_echo` when
appropriate.

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