Don't require IPv6 library support in order to support IPv6 addresses.

Don't require IPv6 library support in order to support IPv6 addresses.

Have our own routines to convert between IPv4/IPv6 addresses and
strings; that helps if, for example, we want to build binary versions of
tcpdump for Windows that can run both on NT 5 (W2K/WXP), which doesn't
have inet_ntop() or inet_pton(), and NT 6 (Vista/7/8/10), which do.  It
also means that we don't require IPv6 library support on UN*X to print
addresses (if somebody wants to build tcpdump for older UN*Xes lacking
IPv6 support in the system library or in add-on libraries).

Get rid of files in the missing directory that we don't need, and
various no-longer-necessary autoconf tests.