Fixing compiler warning and working with musl.
- fix warning about switch fallthrough in parse_isakmp_sa_body() [anatoli]
- fix warning about switch fallthrough in nat_traversal_vid_to_method() [anatoli]
- fix warning about switch fallthrough in finish_pfkey_msg() [anatoli]
- fix warning about switch fallthrough in informational() [anatoli]
- fix warning about switch fallthrough in xauth_inI0() [anatoli]
- lib/libpluto/writehackmsg.c: fix build on musl [Fabrice Fontaine]