Revert "seq_packet: set SOCK_NONBLOCK on accept"

This reverts commit ff22261dde.

This causes issues with lxc blocking on recvmsg.

src/io/seq_packet.rs

@@ -47,12 +47,7 @@ impl SeqPacketListener {
         let fd = self.as_raw_fd();
         let res = self.fd.wrap_read(cx, || {
             c_result!(unsafe {
-                libc::accept4(
-                    fd,
-                    ptr::null_mut(),
-                    ptr::null_mut(),
-                    libc::SOCK_CLOEXEC | libc::SOCK_NONBLOCK,
-                )
+                libc::accept4(fd, ptr::null_mut(), ptr::null_mut(), libc::SOCK_CLOEXEC)
             })
             .map(|fd| unsafe { Fd::from_raw_fd(fd as RawFd) })
         });