Fix assorted compiler warnings.

include/my_pthread.h:
  Like for pthread_cond_timedwait, the abstime is constant.
mysys/my_gethwaddr.c:
  Instead of using a manual copy that introduce warnings due to
  type mismatch, copy the buffer using memcpy and use memcmp to
  check whether all bytes of the buffer are zeroed.
mysys/thr_mutex.c:
  Like for pthread_cond_timedwait, the abstime is constant.
unittest/mytap/tap.h:
  Introduce a ok() variant that does not take a format argument.
  Since ok() is tagged with a printf attribute, GCC complains if
  the fmt argument is NULL.