* Tidying
* Increase test coverage
* Fix file_mode::append_existing
API Changes:
* file_mode::append_new is removed, as it makes no sense
Actions Required:
* Replace file_mode::append_new with file_mode::append
or file_mode::append_existing instead of file_mode::append_new
fix#1356
* File functions return errc::bad_file_descriptor
instead of errc::invalid_argument.
Actions Required:
* Callers checking for errc::invalid_argument from calls to
file APIs should check for errc::bad_file_descriptor instead.