Code review comment for lp:~vkolesnikov/pbxt/pbxt-bug-509803

Fixed a problem in the myxt_create_foreign_key_from_row function. It incorrectly calculated resulting key length which could lead to incorrect inter-table key comparison and false key change detections within a single table. Also perfromaed refactoring: removed XT_KEY_ACTION_DEFAULT that was functionally identical to XT_KEY_ACTION_RESTRICT.