Warn about performance if using TAP-Windows >=9.21.

Warn about performance if using TAP-Windows >=9.21.

Testing has revealed that the newer series of Windows TAP drivers (i.e.
9.0.0.21 and later, also known as NDIS6, tap-windows6) suffer from
serious performance issues in the write path. Write operations seems to
take a very long time to complete, resulting in massive packet loss even
for throughputs as low as 10 Mbit/s.

I've made some attempts to alleviate the problem using parellelism. By
using custom code that allows up to 256 write operations at the same
time the results are much better, but it's still about 2 times worse
than the traditional 9.0.0.9 driver.

We need to investigate more and file a bug against tap-windows6, but in
the mean time, let's inform the user that he might not want to use the
latest drivers.