Modified the check for the timestamp field so that the flags for
the automatic for inserts and updates are cleared independently.


mysql-test/r/type_timestamp.result:
  Bug#7806 - insert on duplicate key and auto-update of timestamp
  The test result.
mysql-test/t/type_timestamp.test:
  Bug#7806 - insert on duplicate key and auto-update of timestamp
  The test case.
sql/mysql_priv.h:
  Bug#7806 - insert on duplicate key and auto-update of timestamp
  Made check_insert_fields() static. It is used only in sql_insert.cc.
sql/sql_insert.cc:
  Bug#7806 - insert on duplicate key and auto-update of timestamp
  Modified the check of the insert fields so that an explicit
  assignment of the timestamp field does only disable the automatic
  for inserts and retains the automatic for updates.
  Added a check if the update fields contain the timestamp field.
  In this case, the automatic on update is disabled, but not the
  automatic on insert. This is called from mysql_prepare_insert().
sql/table.h:
  Bug#7806 - insert on duplicate key and auto-update of timestamp
  Extended a comment to warn about usage of enum timestamp_auto_set_type.