Allow CREATE FUNCTION to run before udf_init()

Currently CREATE FUNCTION requires udf_init() to be called first. But one could get around this restriction by inserting a row into the mysql.func table before a call to udf_init(). Make it so a CREATE FUNCTION statement does the insertion when it is executed before the call to udf_init(). This will for example help with early plugin initialisations (by plugin_load_add) that need to run udf creation queries.