Honor unique/not unique when creating keys for internal tempory tables.
Added new variables to be used to limit how keys are created for internal temporary tables.


include/maria.h:
  Added maria_max_key_length() and maria_max_key_segments()
include/myisam.h:
  Added myisam_max_key_length() and myisam_max_key_segments()
mysql-test/r/mysql.result:
  Drop all used tables
mysql-test/r/subselect4.result:
  Added test case for lp:879939
mysql-test/t/mysql.test:
  Drop all used tables
mysql-test/t/subselect4.test:
  Added test case for lp:879939
sql/mysql_priv.h:
  Added internal_tmp_table_max_key_length and internal_tmp_table_max_key_segments to be used to limit how keys for derived tables are created.
sql/mysqld.cc:
  Added internal_tmp_table_max_key_length and internal_tmp_table_max_key_segments to be used to limit how keys for derived tables are created.
sql/share/errmsg.txt:
  Added new error message for internal errors
sql/sql_select.cc:
  Give error if we try to create a wrong key (this error should never happen)
  Honor unique/not unique when creating keys for internal tempory tables.
storage/maria/ha_maria.cc:
  Added change_table_ptr() to ensure that external_ref points always to the correct table.
  (Not having this caused an assert in the included test)
storage/maria/ha_maria.h:
  Added change_table_ptr() to ensure that external_ref points always to the correct table.
storage/maria/ma_check.c:
  Fixed bug in Duplicate key error printing (now row position is printed correctly)
storage/maria/ma_create.c:
  maria_max_key_length() -> _ma_max_key_length()
storage/maria/ma_info.c:
  Added extern function maria_max_key_length() to calculate the max key length based on current block size.
storage/maria/ma_open.c:
  maria_max_key_length() -> _ma_max_key_length()
storage/maria/maria_def.h:
  maria_max_key_length() -> _ma_max_key_length()
storage/myisam/ha_myisam.cc:
  Added change_table_ptr() to ensure that external_ref points always to the correct table.
  (Not having this caused an assert in the included test)
storage/myisam/ha_myisam.h:
  Added change_table_ptr() to ensure that external_ref points always to the correct table.