Cleanups:
* remove unused HA_EXTRA_EXPORT (can be added later if needed, e.g. for Aria)
* clarify the meaning of HA_CAN_EXPORT
* make all engines that support EXPORT to announce it
* reduce code duplication
Syntax. Server support. Test cases.
InnoDB bugfixes:
* don't mess around with system sprintf's, always use my_error() for errors.
* don't use InnoDB internal error codes where OS error codes are expected.
* don't say "file not found", when it was.
