Clean up 'unused argument' warnings

Add extern "C" { ... } in my_pthread.h




client/mysqlbinlog.cc:
  Include my_pthread.h before thr_alarm.h (otherwise pthread_t isn't
  declared yet).
client/mysqltest.c:
  Fix unused argument warnings.
client/thread_test.c:
  Fix unused argument warnings.
include/my_pthread.h:
  Put extern "C" { ... } wrappers, because this file is included from
  some C++ files.