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.