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: If4e1922f527ba5bfaa0318ac3c93877c7b4924bc