Name resolution for predefined table mapping functions may need to be improved

Right now, predefined table mapping functions like LOG_READER eclipse any user-defined functions with the same name, if the name in the query is unqualified. Example:

create table mapping function trafodion.sch.log_reader ...;

set schema trafodion.sch;
select * from udf(log_reader()); -- uses predefined function
select * from udf(trafodion.sch.log_reader()); -- uses user-defined function

set schema trafodion.othersch;
select * from udf(log_reader()); -- uses predefined function
select * from udf(trafodion.sch.log_reader()); -- uses user-defined function
select * from udf(trafodion.othersch.log_reader()); -- error, UDF not found

This is ANSI standard compliant, I think. The standard says that we need to have a list of schemata to search for the function. In this implementation, the (yet unnamed) schema for predefined functions comes first, then the current default schema. Some people might prefer to search the current default schema first.

Pros for the current solution:
- Easy to understand
- No need to refer to the name of the internal schema of predefined functions explicitly
- Simple workaround if a new predefined function eclipses a TMUDF

Cons of the current solution:
- New predefined functions can potentially eclipse existing functions, though that should be relatively rare