FreeBSD Support within easy reach(?)

[benthor]

I managed to build almost all SCION components under FreeBSD 14.0 with only minor modifications to https://github.com/scionproto/scion/blob/master/private/underlay/conn/conn.go:

Go's syscalls module doesn't define MSG_WAITFORONE for FreeBSD, but per recvmsg(2) it is also a valid flag under FreeBSD. Looking up its value (0x00080000) in /usr/include/sys/socket.h and entering it as a literal worked as a first PoC.

The SCION gateway is the only component that would need a non-trivial patch to work under FreeBSD, since the upstream library to manage TUN/TAP devices only supports Windows, Linux and MacOS.

Overall, FreeBSD Support for SCION seems within easy reach. I can't do it myself at the moment, any takers?

[matzf]

The syscalls.MSG_WAITFORONE bit has been fixed in #4507.