Apply some style(9) to pthread tests

Reported by:	kib
Fixes:		ef135466f8 ("Clean up warnings in pthread tests")
Sponsored by:	The FreeBSD Foundation

tools/regression/pthread/cv_cancel1/cv_cancel1.c

@@ -51,7 +51,8 @@ thr_routine(void *arg __unused)
 	return (NULL);
 }
 
-int main(int argc __unused, char **argv __unused)
+int
+main(void)
 {
 	pthread_t td;
 	int i;

tools/regression/pthread/mutex_isowned_np/mutex_isowned_np.c

@@ -46,7 +46,7 @@ thread(void *arg)
 }
 
 int
-main(int argc __unused, char *argv[] __unused)
+main(void)
 {
 	pthread_t thr;
 	pthread_mutex_t mtx;