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