Changed SHOW ENGINES to work in the same way as I_S.ENGINES.
For this: removed the functions mysqld_show_storage_engines and show_handlerton, and
made SHOW ENGINES work via the common function iter_schema_engines.
There in no test case because an engine (except of MyISAM) may be not compiled or disabled
which may affect the test result.


sql/sql_parse.cc:
  Changed SHOW ENGINES to work in the same way as I_S.ENGINES.
sql/sql_show.cc:
  Changed SHOW ENGINES to work in the same way as I_S.ENGINES.
  For this: removed the functions mysqld_show_storage_engines and show_handlerton, and
  made SHOW ENGINES work via the common function iter_schema_engines.