rp-pppoe: allow MTU to be increased up to 1500

The ethernet data limit on rp-pppoe has been increased to 1508 to
allow an MTU of 1500 to be used. To prevent problems the interface
MTU is checked and used to lower the configured MTU/MRU.

If MIN(MTU/MRU) is > 1492, PPP-Max-Payload is added to PADI and
PADR. If PPP-Max-Payload is received in PADO or PADS, it will be
used to lower the configured MTU/MRU as required.

The MTU/MRU settings are stored and reloaded whenever a connection
is made, to allow for the peer or interface MTU/MRU to increase if
used with persist option.

Conforming to RFC4638, if no PPP-Max-Payload is received, the
negotiated MRU will be limited to 1492.