Add GUC_LIST_INPUT and GUC_LIST_QUOTE to unix_socket_directories

This should have been done in the initial commit that made
unix_socket_directories a list as of c9b0cbe9.  This change allows to
support correctly the case of ALTER SYSTEM, where it is possible to
specify multiple paths as a list, like the following pattern where
flattening is applied to each item:
ALTER SYSTEM SET unix_socket_directories = '/path1', '/path2';

Any parameters specified in postgresql.conf are parsed the same way, so
there is no compatibility change.  pg_dump has a hardcoded list of
parameters marked with GUC_LIST_QUOTE, that gets its routine update.
These are reordered alphabetically for clarity.

Author: Ian Lawrence Barwick
Reviewed-by: Peter Eisentraunt, Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/CAB8KJ=iMOtNY6_sUwV=LQVCJ2zgYHBDyNzVfvE5GN3WQ3v9kQg@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -4173,7 +4173,7 @@ static struct config_string ConfigureNamesString[] =
 		{"unix_socket_directories", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
 			gettext_noop("Sets the directories where Unix-domain sockets will be created."),
 			NULL,
-			GUC_SUPERUSER_ONLY
+			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_SUPERUSER_ONLY
 		},
 		&Unix_socket_directories,
 #ifdef HAVE_UNIX_SOCKETS

src/bin/pg_dump/dumputils.c

@@ -865,11 +865,12 @@ buildACLQueries(PQExpBuffer acl_subquery, PQExpBuffer racl_subquery,
 bool
 variable_is_guc_list_quote(const char *name)
 {
-	if (pg_strcasecmp(name, "temp_tablespaces") == 0 ||
+	if (pg_strcasecmp(name, "local_preload_libraries") == 0 ||
+		pg_strcasecmp(name, "search_path") == 0 ||
 		pg_strcasecmp(name, "session_preload_libraries") == 0 ||
 		pg_strcasecmp(name, "shared_preload_libraries") == 0 ||
-		pg_strcasecmp(name, "local_preload_libraries") == 0 ||
-		pg_strcasecmp(name, "search_path") == 0)
+		pg_strcasecmp(name, "temp_tablespaces") == 0 ||
+		pg_strcasecmp(name, "unix_socket_directories") == 0)
 		return true;
 	else
 		return false;