Add basic OpenBSD support

XXX more work needed to manage VT switches and proper
XXX fbtab(4) support to allow for non-root users

tests/poller.c

@@ -123,6 +123,10 @@ static int test_signal_event(int signal, void *data) {
 	return 0;
 }
 
+#ifdef __OpenBSD__
+#define SIGRTMIN  SIGUSR1
+#endif
+
 static void test_poller_single_signal(void) {
 	struct poller poller;
 	test_assert(poller_init(&poller) == 0);