message: remove obsolete interface between lwip and libhermit
diff: use arch;
  use arch::percore::*;
  use console;
  -use core::u32;
  use synch::spinlock::SpinlockIrqSaveGuard;
  -use syscalls::tasks::Tid;
  /// Enables lwIP's printf to print a whole string without being interrupted by
  /// a message from the kernel.
  static mut CONSOLE_GUARD: Option<SpinlockIrqSaveGuard<console::Console>> = None;
  -/// Task ID of the single TCP/IP Task spawned by lwIP.
  -/// Initialized to u32::MAX by default, which is a very unlikely task ID.
  -static mut LWIP_TCPIP_TASK_ID: Tid = u32::MAX;
  -
  -pub fn get_lwip_tcpip_task_id() -> Tid {
  - unsafe { LWIP_TCPIP_TASK_ID }
  -}
  -
  -#[no_mangle]
  -pub extern "C" fn sys_lwip_register_tcpip_task(id: Tid) {
  - unsafe {
  - LWIP_TCPIP_TASK_ID = id;
  - }
  -}
  -
  #[no_mangle]
  pub extern "C" fn sys_lwip_get_errno() -> i32 {
  let lwip_errno = core_scheduler().current_task.borrow().lwip_errno;
  