Moved <netinet6/in6.h> special processing to IPv6 section. Changed
the CPP symbol it uses from *_HAVENETINET6* to *_NEEDNETINET6* to better clarify what's going on with regard to the symbol. Moved processing of --with-kame to before the in6addr_any check, since it needs to set LIBS for the check to work. [related to RT #105, and previous configure.in revision 1.108.] in6_pkinfo check only needs AC_TRY_COMPILE, not AC_TRY_LINK.