l2tp/ppp: align with Android 4.4 userspace

Needed to fix mtpd (vpn) functionality.

See: https://groups.google.com/forum/#!msg/android-x86/1uCHFx4glgc/Yis3PkrD-nwJ

Change-Id: I8cf33c889dabcc7db428dce8479e760e5a9d5ad1