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.