Post-review fixes that simplify the way access rights
are checked during name resolution and factor out all
entry points to check access rights into one single
function.


sql/item.cc:
  Simplfied find_field_in_table - factored out all acces right checks
  into a separate function.
sql/mysql_priv.h:
  Simplified the way we control whether to perform access right
  checks for columns.
sql/sql_acl.cc:
  - Added new functon check_column_grant_in_table_ref that serves as a single
    point of entry to check access rights during name resolution for different
    kinds of table references.
  - Moved check_grant_column_in_sctx to sql_acl.cc where it
    logically belongs.
  - Removed the parameter check_grants - it is checked before
    calling the function.
sql/sql_acl.h:
  - Added new function check_column_grant_in_table_ref.
  - Made check_grant_column_in_sctx available to other modules.
sql/sql_base.cc:
  - Factored out all code that check access rights for columns during
    name resolution into one function - check_column_grant_in_table_ref.
  - Moved check_grant_column_in_sctx to sql_acl.cc where it
    logically belongs.
  - Removed the parameter check_grants - it is checked before
    calling the function.
sql/table.cc:
  Removed code that duplicates the functionality of
  check_column_grant_in_table_ref, and called directly
  that function.
sql/table.h:
  check_grants method is replaced by more general
  check_column_grant_in_table_ref.