Previously, these drivers would check /sys/devices/virtual/misc/tun to
see if TUN is supported, and bail out otherwise. However, the tun module
can sometimes be autoloaded by opening the /dev/net/tun file. In this
case the /dev file already exists, but the /sys file only gets created
after the modul is loaded.
Additionally, the ppp code does not use the /sys file, but only the
/dev file, so checking for the existence of the latter seems a better
indicator of expected success.