DragonFlyBSD

Hi Everyone,

I'm testing some software on DragonFly. There's not much to it. It
talks to the modem. It sends an ATZ and reads the response. Linux,
FreeBSD, NetBSD, OpenBSD and OS X are OK.

For my test rig I have a USR5637 modem,
https://www.amazon.com/gp/product/B0013FDLM0. The modem is located at
/dev/cuaU0.

DragonFly hangs on the call to tcdrain(3). Looking at the man page I
don't see any special handling. Cf.,
http://man.dragonflybsd.org/?command=tcdrain.

Attached is the reproducer. The trace is:

% ./test.exe
Setting TIOCEXCL
Getting tty
Setting tty options
Flushing tty
Setting tty
Writing ATZ
Waiting for write <<-- call to tcdrain(fd)

Thanks.