causes out of memory errors

The code in mysql_create_function() and mysql_drop_function() assumed
that the only reason for UDFs being uninitialized at that point is an
out-of-memory error during initialization. However, another possible 
reason for that is the --skip-grant-tables option in which case UDF 
initialization is skipped and UDFs are unavailable.

The solution is to check whether mysqld is running with
--skip-grant-tables and issue a proper error in such a case.


mysql-test/r/skip_grants.result:
  Added a test case for bug #32020.
mysql-test/t/skip_grants.test:
  Added a test case for bug #32020.
sql/sql_udf.cc:
  Issue a proper error when a user tries to CREATE/DROP a UDF 
  on a server running with the --skip-grant-tables option.