pgsql: Remove configure probe for sockaddr_in6 and require AF_INET6.

Remove configure probe for sockaddr_in6 and require AF_INET6.

SUSv3 <netinet/in.h> defines struct sockaddr_in6, and all targeted Unix
systems have it. Windows has it in <ws2ipdef.h>. Remove the configure
probe, the macro and a small amount of dead code.

Also remove a mention of IPv6-less builds from the documentation, since
there aren't any.

This is similar to commits f5580882 and 077bf2f2 for Unix sockets. Even
though AF_INET6 is an "optional" component of SUSv3, there are no known
modern operating system without it, and it seems even less likely to be
omitted from future systems than AF_UNIX.

Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bcc8b14ef630b2ad9aae7813981fb248fbff9ed8

Modified Files
--------------
configure | 10 ----------
configure.ac | 6 ------
doc/src/sgml/client-auth.sgml | 2 --
src/backend/libpq/auth.c | 21 ---------------------
src/backend/libpq/hba.c | 5 -----
src/backend/libpq/ifaddr.c | 18 +-----------------
src/backend/libpq/pqcomm.c | 2 --
src/backend/utils/adt/network.c | 10 ----------
src/backend/utils/adt/pgstatfuncs.c | 11 ++---------
src/bin/initdb/initdb.c | 10 ----------
src/include/pg_config.h.in | 3 ---
src/include/utils/inet.h | 4 ++--
src/interfaces/libpq/fe-connect.c | 2 --
src/port/inet_net_ntop.c | 5 ++---
src/tools/ifaddrs/test_ifaddrs.c | 2 --
src/tools/msvc/Solution.pm | 1 -
16 files changed, 7 insertions(+), 105 deletions(-)