Fix tests to pass on newer versions of scapy.

1. New versions of scapy no longer use the "unused" field to
   indicate the MTU in an IPv4 fragmentation needed packet,
   correctly calling it "nexthopmtu" instead. Add code to
   support both old and new versions.
2. Replace "load" with "payload" in the null encryption code.

Bug: 36510518
Test: IPv4 PMTU tests pass on device
Test: null encryption xfrm tests pass on device
Test: all_tests.sh pass on android-4.9
Change-Id: Id414080d06ac9ffa66aa5053f6dcc5e5869060a9