Consider adding socketopt const "IPV6_BOUND_IF" as we need to set/get bound interface on macOS

[KyleRicardo]

Consider add this to bsd/apple/mod.rs:

pub const IPV6_BOUND_IF: ::c_int = 125;

We really need this constant to perform modification on socket options(to bind socket to an interface, more specifically). Though we can temporarily define this on our own, but since IP_BOUND_IF is already defined in libc crate, we'd like to keep consistency.
I can make a simple PR if you agree to add this.
Thanks for your hard work.

[KyleRicardo]

5 days passed. Any feedback?

[thomcc]

If it's in the headers I don't see a reason why a PR wouldn't be accepted.

[devnexen]

@KyleRicardo for next time, you do not need "permission" if you think you can do a PR.

[KyleRicardo]

@devnexen Ok, next time I'll do a PR directly. BTW thanks for your PR. Hopefully it will be merged soon.