VPP-92 Fixup some srv6 issues

VPP-92 Fixup some srv6 issues

Some small fixes to the srv6 code to bring it toward
conformance with draft-previdi-6man-segment-routing-header-05.

- The first segment needs to remain in the segment list.
- The segment list template needs a space for the ultimate
  destination.
- The ultimate destination needs to be inserted into that
  space when adding the SRH.

Change-Id: I66db6912e0128da084f14ceca20918ef67ccff79
Signed-off-by: Chris Luke <chrisy@flirble.org>
(cherry picked from commit 4b8b718ff9bbba3bea018505ac591bb737f68001)