In fact, the code was already doing this via three compile-time
conditionals (ISC_PLATFORM_HAVEIPV6, ISC_PLATFORM_HAVEIN6PKTINFO,
ISC_PLATFORM_HAVESCOPEID) that were always on.  This behavior, in
turn, was inherited from the NTP Classic autoconf build.

Cleaning this up lets us drop a substantial amount of shim
code in libntp/ntp_rfc2553.[ch].