Add support for O_DIRECT

[laurynas-biveinis]

O_DIRECT is documented to "Try to minimize cache effects of the I/O to and from this file.", thus it might be doing the opposite in libeatmydata might be desirable. A challenge would be to check for alignment / block size violations, for which EINVAL should be returned instead of performing the I/O in buffered mode.

[laurynas-biveinis]

(the implementation should include fcntl(fd, F_SETFL, O_DIRECT) support too, not only open flag)