btest is Mikrotik's bandwidth test protocol; they only provide the
RouterOS implementation ("/tool bandwidth-test" and a Windows binary
btest.exe but no documentation or source.

This is a reverse-engineered (partially) interoperable implementation.

Currently on OpenBSD it only seems to work properly when run as a
server in TCP mode. The client can set the direction of transfer;
on OpenBSD speeds seem accurate when btest-opensource sends to
a RouterOS client, but speeds for received packets seem wrong;
it expects clock_nanosleep() which is not available on OpenBSD
and perhaps the workaround is not good enough.
