Add FreeBSD BPF structures

[jbit]

This PR adds BPF structures that are common between FreeBSD 11, FreeBSD 12, FreeBSD mainline/13 and DragonFlyBSD 5.9.

It also fixes the definition of BPF_ALIGNMENT, which should be equal to sizeof(long), which is 4 on 32bit FreeBSD.

https://www.freebsd.org/cgi/man.cgi?query=arch&sektion=7

All supported ABIs can be divided into two groups:

• ILP32: int, long, void * types machine representations all have 4-byte size.
• LP64: int type machine representation uses 4 bytes, while long and void * are 8 bytes.

I introduced the private SIZEOF_LONG const, since I didn't want to introduce a dependency on rust 1.24+ by depending on libc_const_size_of.

These changes allow my experimental crate to build on FreeBSD: https://github.com/jbit/powerline

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[JohnTitor]

Looks good, thanks!

[JohnTitor]

@bors r+

[bors]

📌 Commit 4b55c87 has been approved by JohnTitor

[bors]

⌛ Testing commit 4b55c87 with merge 83d8c39...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 83d8c39 to master...