/lib/network/ppp.sh fixes a problem with PPtP (and other PPP-based things) where...

/lib/network/ppp.sh fixes a problem with PPtP (and other PPP-based things) where PPtP + PPPoE will fail to work. This is because PPtP defaults to ppp0 and so does PPPoE, because /etc/init.d/ppp reinitializes the unit counter for each PPP-type connection (so you could have multiple PPtP over cable, for instance, but PPPoE + PPtP is a problem).

/etc/init.d/ppp automatically starts pptp session on boot., from #6720

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@20221 3c298f89-4303-0410-b956-a3cf2f4a3e73