The problem was that the grammar allows to create a function with an optional
definer clause, and define it as a UDF with the SONAME keyword.
Such combination should be reported as an error.

The solution is to not change the grammar itself, and to introduce a
specific check in the yacc actions in 'create_function_tail' for UDF,
that now reports ER_WRONG_USAGE when using both DEFINER and SONAME.


mysql-test/r/udf.result:
  Added tests for Bug#21269 (DEFINER-clause is allowed for UDF-functions)
mysql-test/t/udf.test:
  Added tests for Bug#21269 (DEFINER-clause is allowed for UDF-functions)
sql/sql_yacc.yy:
  Creating a UDF function with a DEFINER clause is now a syntax error.