net: move PPPoLAC and PPPoPNS headers to uapi

Move the entire contents of the linux/if_pppolac.h and
linux/if_pppopns.h headers to uapi, they only contain userspace
interfaces.

Change-Id: I3cfed7f2ae400b53269a1f59144aa3dbc30ae0b5
Signed-off-by: Colin Cross <ccross@android.com>