net/mpd5: add L2TP fix from upstream (r2410, r2411).

This change should fix the problem reported in bugs #38 #47 #74.
It is L2TP interoperability issue with some Juniper, Cisco and
YAMAHA RTX routers having strict protocol checks.

Move USES to pet portlint.

Reported by: Kikuchan at SourceForge (based on)