Use temporary constraint names for temporary tables. The constraints
are not added to cache (skipped in dict_table_rename_in_cache()).

The scheme for temporary constraint names is as follows:

    for old table: db_name/\xFFconstraint_name
    for new table: db_name/\xFF\xFFconstraint_name

normalize_table_name_c_low(): wrong comparison "less than FN_REFLEN -
1". Somewhere array of FN_REFLEN includes the trailing 0, somewhere
array of FN_REFLEN + 1 includes trailing 0, but nowhere array of
FN_REFLEN - 1 must include trailing 0.