Detect OS IPv6 support using AF_INET6 only.

Detect OS IPv6 support using AF_INET6 only.

tcpdump source code has not been using struct in6_addr since commit
0c9cfdc in 2019, so lose the conditional structure declaration, which is
a no-op.

Since commit de7c619 in 2015 netdissect-stdinc.h on Windows defines
HAVE_OS_IPV6_SUPPORT if AF_INET6 if defined, which makes it equivalent
to AF_INET6.  On Unix-like systems taking struct in6_addr out of scope
would make HAVE_OS_IPV6_SUPPORT equivalent to AF_INET6, thus after
removing struct in6_addr remove HAVE_OS_IPV6_SUPPORT together with
Autoconf and CMake checks that define it.  Leave an unrelated CMake
workaround in place for later debugging.

On Windows do not define AF_INET6 if it is not defined, which makes
AF_INET6 a universal indicator of the OS IPv6 support on all supported
OSes.  The few remaining use cases that genuinely need AF_INET6 use it
to make OS API calls, so if the macro is not defined, it most likely
means such an API call in the best case would return just a well-formed
error status.  With this in mind, in win32_gethostbyaddr() and
ip6addr_string() guard all IPv6-specific code with #ifdef AF_INET6.  In
tcpdump.c add a comment to note why a guard is not required for
Casper-specific conditional code that uses AF_INET6.

This way when the OS does not support IPv6, IPv6 addresses will not
resolve to names, which is expected.  Other than that, tcpdump should be
able to process IPv6 addresses the usual way regardless if the OS would
be able to process the packets with these addresses.