Fix some usability issues + parse flags from libc sources if possible

[chhh]

Turned out flags might differ on some systems, should be a benign change

[smacke]

Hey, thanks for the PR. I'm a bit concerned about spawning a new process like this for parsing out the various flags -- see my comment on https://github.com/smacke/jaydio/pull/7. What are the OSes that support O_DIRECT that could have different values for these flags? Is there a way we could get this information without spawning a new process inside a static context, and include the original defaults as fallbacks?