Remove our dependencies on MB_CUR_MAX in favor of believing that

pg_database_encoding_max_length() predicts the maximum character length
returned by wchar2char().  Per Hiroshi Inoue, MB_CUR_MAX isn't usable on
Windows because we allow encoding = UTF8 when the locale says differently;
and getting rid of it seems a good idea on general principles because it
narrows our dependence on libc's locale API just a little bit more.

Also install a check for overflow of the buffer size computation.