Change the built-in default socket directory to be /var/run/postgresql.
For backwards compatibility with (probably non-libpq-based) clients that
might still expect to find the socket in /tmp, also create a socket in
/tmp.  This is to resolve communication problems with clients operating
under systemd's PrivateTmp environment, which won't be using the same
global /tmp directory as the server; see bug #825448.

Note that we apply the socket directory change at the level of the
hard-wired defaults in the C code, not by just twiddling the setting in
postgresql.conf.sample; this is so that the change will take effect on
server package update, without requiring any existing postgresql.conf
to be updated.  (Of course, a user who dislikes this behavior can still
override it via postgresql.conf.)

diff -up postgresql-13.0/src/include/pg_config_manual.h.patch1 postgresql-13.0/src/include/pg_config_manual.h
--- postgresql-13.0/src/include/pg_config_manual.h.patch1	2020-10-23 10:18:12.204959024 +0200
+++ postgresql-13.0/src/include/pg_config_manual.h	2020-10-23 10:18:39.391208931 +0200
@@ -201,7 +201,7 @@
  * support them yet.
  */
 #ifndef WIN32
-#define DEFAULT_PGSOCKET_DIR  "/tmp"
+#define DEFAULT_PGSOCKET_DIR  "/var/run/postgresql"
 #else
 #define DEFAULT_PGSOCKET_DIR ""
 #endif