build 1 (stf_race.c):   cc -O2 -pthread -o /tmp/stf_race /tmp/stf_race.c
  -rwxr-xr-x  1 root  wheel  10632 Sep 12 15:19 /tmp/stf_race
build 2 (stf_race.c fix): REBUILT
build 3 (stf_async_race.c): cc -O2 -pthread -o /tmp/stf_async_race /tmp/stf_async_race.c
  BUILT (first version: single-thread fd open raced /dev/tap0 single-opener
  semantics -> "open /dev/tap0: Device busy" for the 2nd thread; fixed by
  sharing one fd; MAC discovery replaced with broadcast dst MAC)
build 4 (final): BUILT