pgsql: Remove our dependencies on MB_CUR_MAX in favor of believing that

Log Message:
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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.

Modified Files:
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    pgsql/src/backend/utils/adt:
        formatting.c (r1.151 -> r1.152)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c?r1=1.151&r2=1.152)