Add new sendfile flags for FreeBSD

[morrowa]

Two new constants for FreeBSD:

• SF_USER_READAHEAD introduced in FreeBSD 12 in this commit (omitted from tests until FreeBSD 12 is stable)
• SF_NOCACHE introduced in FreeBSD 11 in this commit

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 96ee7bf has been approved by alexcrichton

[bors]

⌛ Testing commit 96ee7bf with merge f464f1b...

[bors]

💔 Test failed - status-travis

[morrowa]

• 3864.8 failed waiting on an interactive prompt for an Android SDK license agreement (oops!).
• 3864.16 appears to have panicked without a backtrace. I'll try running the tests locally on my Mac with the same configuration.
• 3864.34 timed out with no output for 10 minutes. I'm most concerned about this since it's a FreeBSD test and that's what I changed.

[alexcrichton]

@bors: retry

Hm hopefully spurious!

[bors]

⌛ Testing commit 96ee7bf with merge 5e025d6...

[morrowa]

OK, looks like the only mandatory failure now is the Android build which still needs help to auto-accept the license agreement.

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 5e025d6 to master...