Revert removal of trace_userlocks, because userlocks aren't gone.

This reverts commit 0180bd6180.
contrib/userlock is gone, but user-level locking still exists,
and is exposed via the pg_advisory* family of functions.

src/backend/utils/misc/check_guc

@@ -20,7 +20,7 @@ INTENTIONALLY_NOT_INCLUDED="autocommit debug_deadlocks \
 is_superuser lc_collate lc_ctype lc_messages lc_monetary lc_numeric lc_time \
 pre_auth_delay role seed server_encoding server_version server_version_int \
 session_authorization trace_lock_oidmin trace_lock_table trace_locks trace_lwlocks \
-trace_notify transaction_isolation transaction_read_only \
+trace_notify trace_userlocks transaction_isolation transaction_read_only \
 zero_damaged_pages"
 
 ### What options are listed in postgresql.conf.sample, but don't appear

src/backend/utils/misc/guc.c

@@ -1060,6 +1060,16 @@ static struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+	{
+		{"trace_userlocks", PGC_SUSET, DEVELOPER_OPTIONS,
+			gettext_noop("No description available."),
+			NULL,
+			GUC_NOT_IN_SAMPLE
+		},
+		&Trace_userlocks,
+		false,
+		NULL, NULL, NULL
+	},
 	{
 		{"trace_lwlocks", PGC_SUSET, DEVELOPER_OPTIONS,
 			gettext_noop("No description available."),