- more tests, post-review changes, bug-fixes, simplifications, and improved comments


mysql-test/r/having.result:
  - added tests for subqueries with HAVING
  - added tests for few other discovered bugs
  - renamed tables to the more standard t1,t2,..
mysql-test/t/having.test:
  - added tests for subqueries with HAVING
  - added tests for few other discovered bugs
  - renamed tables to the more standard t1,t2,..
sql/item.cc:
  - Extended the name resolution to support nested HAVING clauses in nested sub-queries
  - Factored out the code that resolves a column ref against a single query
  - Fixed several logical bugs
  - Removed unused variables
  - More/better comments
sql/sql_base.cc:
  Corrected function spec.