Use wcstombs and friends to perform Unicode conversion inside the ODBC
driver. Functionality should be identical as before, except that
installations without iconv will now be able to compile and install the
Unicode (libmdbodbcW) driver.
iconv remains present in the main libmdb library.
Add new mdbi_register_backend2 function with the features introduced in
PRs #321 and #322. The purpose is to prevent API/ABI breakage in the
1.0 release.