Fails to build on i386: undefined: syscall.SYS_SETSOCKOPT

[siretart]

When trying to build v0.4.2 on i386, I get the following build error:

github.com/rootless-containers/bypass4netns/pkg/bypass4netns
# github.com/rootless-containers/bypass4netns/pkg/bypass4netns
src/github.com/rootless-containers/bypass4netns/pkg/bypass4netns/socket.go:433:43: undefined: syscall.SYS_SETSOCKOPT

doesn't happen on arm or 64bit.

Actually, this also occurred on v0.4.1

[AkihiroSuda]

Do we still need to support i386?

https://www.debian.org/releases/stable/release-notes/issues.en.html#i386-reduced-support