Fix pptp handling of routes to server.

Fix pptp handling of routes to server.

The existing code is fairly broken. It assumes you're using Legacy IP, and
it assumes that the server is reachable via your default route. Via the
first default route in the 'route -n' output, in fact, regardless of metric.

Fix all those problems by using 'ip route get' to really find the *current*
route to the server, and install a host-specific route to match.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31565 3c298f89-4303-0410-b956-a3cf2f4a3e73