Adding SCTP support (Linux/FreeBSD)

[nathaniel-bennett]

I'm looking at adding SCTP support to the socket2 crate, and it would be useful to have definitions of constants and structs from netinet/sctp.h to do so. It seems like this crate is the most appropriate place to have those defined, given that definitions for similar protocols (TCP/UDP, or even the Linux-specific Netlink) are found here.

I would be happy to add the appropriate bindings and submit a PR for it here; I just wanted to check to see if such an addition would be desired.

[devnexen]

I would suggest to submit a PR, makes sense.