In a dual-WAN setup, it's useful to specify an interface over which to have PPTP. Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us> git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44507 3c298f89-4303-0410-b956-a3cf2f4a3e73