libtoolize: refactor $opt_quiet, func_echo and func_echo_once.

libtoolize: refactor $opt_quiet, func_echo and func_echo_once.

Instead of long sequences checking for setting of $opt_quiet,
followed by one or more func_echo variations, add and use a new
function func_whisper - the complement of func_verbose.
* libtoolize.m4sh (func_whisper): New function. Adjust all
callers previously using `$opt_quiet || func_echo ...'.
(func_whisper_once): Ditto, for `$opt_quiet || func_echo_once
...'.
(func_whisper_hdr): New function, adjust all callers previously
using `func_whisper_once ...; func_whisper ...'.
(func_whisper_error_header): Ditto, but calling func_error.
(func_echo_once): Remove.

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