Using /tmp for sockets allows everyone to spoof a PostgreSQL server. Thus use
/run/postgresql/ for "system" clusters which run as 'postgres' (user
clusters will still use /tmp). Since system cluster are by far the common case,
set it as default.

This is inspired by Fedora and Debian patches.

--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -63,7 +63,7 @@
 #port = 5432				# (change requires restart)
 #max_connections = 100			# (change requires restart)
 #superuser_reserved_connections = 3	# (change requires restart)
-#unix_socket_directories = '/tmp'	# comma-separated list of directories
+unix_socket_directories = '/run/postgresql'	# comma-separated list of directories
 					# (change requires restart)
 #unix_socket_group = ''			# (change requires restart)
 #unix_socket_permissions = 0777		# begin with 0 to use octal notation
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -201,7 +201,7 @@
  * support them yet.
  */
 #ifndef WIN32
-#define DEFAULT_PGSOCKET_DIR  "/tmp"
+#define DEFAULT_PGSOCKET_DIR  "/run/postgresql"
 #else
 #define DEFAULT_PGSOCKET_DIR ""
 #endif