fixing test results accordingly.
func_system.test:
  New test that illegal mix of collations does not happen anymore.
item_strfunc.h:
    safe_charset_converter() was added for system constants.
item_strfunc.cc:
  safe_charset_converter() was added for system constants.
item_func.cc, item.h, item.cc:
  Bug#8291: Illegal collation mix with USER() function.
  After discussion with PeterG and Serge, a new coercibility
  level for "system constants" was introduced, between
  COERRIBLE and IMPLICIT. Thus:
  SELECT col1 = USER() FROM t1; - is done according to col1 collation.
  SELECT 'string' = USER(); - is done according to USER() collation.
  At the same time, "nagg" and "strong" members were removed as unused.
item_create.cc:
  Version is a system constant too.