Do type conversion to match columns in UNION clauses.

Do type conversion to match columns in UNION clauses.
 Currently force the type to match the _first_ select in the union.
Move oper_select_candidate() from parse_func.c to parse_oper.c.
Throw error inside of oper_inexact() if no match for binary operators.
Check more carefully that types can be coerced
 even if there is only one candidate operator in oper_inexact().
Fix up error messages for more uniform look.
Remove unused code.
Fix up comments.