Only read from TAP-Win32 if the device is enabled.

With newer TAP-Win32 versions (such as the experimental
tap-windows6 9.21.0), tinc is unable to read from the virtual network
device:

    Error while reading from (null) {23810A13-BCA9-44CE-94C6-9AEDFBF85736}: No such file or directory

This is because these new drivers apparently don't accept reads when
the device is not in the connected state (media status).

This commit fixes the issue by making sure we start reading no sooner
than when the device is enabled, and that we stop reading when the
device is disabled. This also makes the behavior somewhat cleaner,
because it doesn't make much sense to read from a disabled device
anyway.
Handle TAP-Win32 immediate reads correctly.

The handling of TAP-Win32 virtual network device reads that complete
immediately (ReadFile() returns TRUE) is incorrect - instead of
starting a new read, tinc will continue listening for the overlapped
read completion event which will never fire. As a result, tinc stops
receiving packets on the interface.