pgsql: Remove configure probes for sys/un.h and struct sockaddr_un.

Remove configure probes for sys/un.h and struct sockaddr_un.

<sys/un.h> is in SUSv3 and every targeted Unix has it. Some Windows
tool chains may still lack the approximately equivalent header
<afunix.h>, so we already defined struct sockaddr_un ourselves on that
OS for now. To harmonize things a bit, move our definition into a new
header src/include/port/win32/sys/un.h.

HAVE_UNIX_SOCKETS is now defined unconditionally. We migh remove that
in a separate commit, pending discussion.

Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BL_3brvh%3D8e0BW_VfX9h7MtwgN%3DnFHP5o7X2oZucY9dg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/077bf2f2750076675ae7195c58bec9a62d62fd34

Modified Files
--------------
config/c-library.m4 | 14 --------------
configure | 17 +----------------
configure.ac | 2 --
src/include/c.h | 4 ----
src/include/libpq/pqcomm.h | 4 +---
src/include/pg_config.h.in | 6 ------
src/include/port.h | 3 +++
src/include/port/win32.h | 11 -----------
src/include/port/win32/sys/un.h | 17 +++++++++++++++++
src/port/getpeereid.c | 4 +---
src/tools/msvc/Solution.pm | 2 --
11 files changed, 23 insertions(+), 61 deletions(-)