ppp: allow auto-detecting and creation of ipv6 subinterface

this makes ipv6 with ppp a bit more comfortable

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42854 3c298f89-4303-0410-b956-a3cf2f4a3e73