message: Fix build on systems where socklen_t is signed
diff: @@ -127,7 +127,7 @@ pub fn channel(network_interface: &NetworkInterface, config: Config) -> io::Resu
  linux::SOL_PACKET,
  linux::PACKET_ADD_MEMBERSHIP,
  (&pmr as *const linux::packet_mreq) as *const libc::c_void,
  - mem::size_of::<linux::packet_mreq>() as u32,
  + mem::size_of::<linux::packet_mreq>() as libc::socklen_t,
  )
  } == -1
  {
  @@ -169,7 +169,7 @@ pub fn channel(network_interface: &NetworkInterface, config: Config) -> io::Resu
  linux::SOL_PACKET,
  linux::PACKET_FANOUT,
  (&arg as *const libc::c_uint) as *const libc::c_void,
  - mem::size_of::<libc::c_uint>() as u32,
  + mem::size_of::<libc::c_uint>() as libc::socklen_t,
  )
  } == -1
  {
  