Define a locking regime for the members of struct timerfd and document
it so future code can follow the standard. The lock legend can be found
in a comment above struct timerfd.

Additionally,
* Add assertions based on locking regime.
* Fill kn_data with the expiration count when EVFILT_READ is triggered.
* Report st_ctim for stat(2).
* Check if file has f_type == DTYPE_TIMERFD before assigning timerfd
  pointer to f_data.