Warn if using unnamed diversion.

Warn if using unnamed diversion.

* lib/m4sugar/m4sugar.m4 (_m4_divert, m4_divert_push): Add
optional parameter, which controls warning.
(m4_divert_pop, m4_cleardivert, m4_divert_require)
(_m4_require_call): Adjust callers.
* lib/m4sugar/m4sh.m4 (AS_REQUIRE): Likewise.
* tests/m4sh.at (AT_DATA_LINENO): Avoid triggering the warning.
* tests/m4sugar.at (AT_CHECK_M4SUGAR_TEXT, m4@&t@_append)
(m4@&t@_text_wrap, recursion): Likewise.
(m4@&t@_warn, m4@&t@_divert_stack): Adjust expected output.
* tests/tools.at (autom4te and whitespace in file names)
(autoconf: the empty token): Avoid triggering the warning.
(autoconf: AC_PRESERVE_HELP_ORDER): New test.
* tests/mktests.sh (ac_exclude_list): Retire prior test.
* NEWS: Document the warning.
* doc/autoconf.texi (Redefined M4 Macros) <m4_divert>,
<m4_undivert>: Make even more explicit that using these directly
is discouraged.
(Diversion support): Further warn against improper diversion
changes.
<m4_divert_text>: Give an example of proper use.
Reported by Mike Frysinger.

Signed-off-by: Eric Blake <ebb9@byu.net>