Use a mutex to allow the TAP reader to process packets faster on Windows.

The TAP-Win32 device is not a socket, and select() under Windows only works
with sockets.  Tinc used a separate thread to read from the TAP-Win32 device,
and passed this via a local socket to the main thread which could then select()
from it. We now use a global mutex, which is only unlocked when the main thread
is waiting for select(), to allow the TAP reader thread to process packets
directly.