Add hooks in base cxgbe(4) for the iWARP upper-layer driver.  Update a
couple of assertions in the TOE driver as well.
Change t4_list_lock and t4_uld_list_lock from mutexes to sx'es.

- tom_uninit had to be reworked not to hold the adapter lock (a mutex)
  around t4_deactivate_uld, which acquires the uld_list_lock.
- the ifc_match for the interface cloner that creates the tracer ifnet
  had to be reworked as the kernel calls ifc_match with the global
  if_cloners_mtx held.
libutil: Use O_CLOEXEC for internal file descriptors from open().