This patch changes the main name of 3 byte character set from utf8 to
utf8mb3. New old_mode UTF8_IS_UTF8MB3 is added and set TRUE by default,
so that utf8 would mean utf8mb3. If not set, utf8 would mean utf8mb4.
* It uses stack a lot, limit the recursion depth like
we used to do for pcre.
* But don't do that for newer pcre2 that uses stack very little
(and doesn't support limiting recursion depth anyway).
* Fix tests to verify only that deep recursion doesn't crash, but
ignore results and warnings (which depend on pcre2 version)
* Also different pcre2 versions report different offset in error messages,
take that into account.
