- Changed name resolution for GROUP BY so that derived columns do not shadow table columns
  from the FROM clause. As a result GROUP BY now is handled as a true ANSI extentsion.
- Issue a warning when HAVING is resolved into ambiguous columns, and prefer the columns from
  the GROUP BY clause over SELECT columns.


mysql-test/r/having.result:
  Correct result for updated GROUP BY name resolution.
sql/item.cc:
  - prefer GROUP columns, but if none is found use SELECT list
  - issue a waring when a field may be resolved ambiguously
  - more/fixed comments
sql/mysql_priv.h:
  More flexible find_field_in_tables().
sql/sp.cc:
  More flexible find_field_in_tables().
sql/sql_base.cc:
  More flexible find_field_in_tables().
sql/sql_help.cc:
  More flexible find_field_in_tables().
sql/sql_select.cc:
  - name resolution of GROUP/ORDER BY column references is differentiated:
    - GROUP BY is resolved in SELECT and FROM clauses
    - ORDER BY is resolved only in SELECT (as before)
  - more informative variable names
  - more comments