Add select

[alexispires]

Allow select to this kind of design:

for i := 1; i > 0; i, _ = conn.Select(&unix.Timeval{}) {
	rmsg, err := conn.Receive()

	if err != nil {
		return fmt.Errorf("Receive: %w", err)
	}

	for _, msg := range rmsg {
		// do something
	}
}

I need this to trying reproduce behaviors of nftables which check with select before doing a recvmsg, example of strace:

select(4, [3], NULL, NULL, {tv_sec=0, tv_usec=0}) = 1 (in [3], left {tv_sec=0, tv_usec=0})
recvmsg(......)
select(4, [3], NULL, NULL, {tv_sec=0, tv_usec=0}) = 1 (in [3], left {tv_sec=0, tv_usec=0})
recvmsg(......)
select(4, [3], NULL, NULL, {tv_sec=0, tv_usec=0}) = 0 (Timeout)

[mdlayher]

I think it makes more sense to use an API like SyscallConn for this, although I'm also not sure why you wouldn't just let receive wait on the socket since the runtime handles it in an efficient way.

https://godoc.org/github.com/mdlayher/netlink#Conn.SyscallConn

[alexispires]

I think it makes more sense to use an API like SyscallConn for this, although I'm also not sure why you wouldn't just let receive wait on the socket since the runtime handles it in an efficient way.

https://godoc.org/github.com/mdlayher/netlink#Conn.SyscallConn

Ok it works by using SyscallConn.

I need this to prevent to be blocked by recvmsg because the func which make the syscall need to return some results. As nftables:
http://git.netfilter.org/nftables/tree/src/mnl.c#n349

I think it's better to have it directly in this lib but you can close this PR if you want ;)

[alexispires]

Related to google/nftables#88

[mdlayher]

It could be worth pursuing something like this as an internal change to unblock issues like #136, but I don't want to expose this API directly. Will close this, but thanks for your time.