add a pthread_get_name_np to match pthread_set_name_np.

could be useful in ports.
initial diff by David Carlier some time ago.
ok jca

src/include/pthread_np.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: pthread_np.h,v 1.1 2017/10/15 23:40:33 guenther Exp $	*/
+/*	$OpenBSD: pthread_np.h,v 1.2 2019/02/04 17:18:08 tedu Exp $	*/
 /*
  * Copyright (c) 1996-98 John Birrell <jb@cimlogic.com.au>.
  * All rights reserved.
@@ -45,6 +45,7 @@ typedef void	(*pthread_switch_routine_t)(pthread_t, pthread_t);
 __BEGIN_DECLS
 int pthread_mutexattr_getkind_np(pthread_mutexattr_t);
 int pthread_mutexattr_setkind_np(pthread_mutexattr_t *, int);
+void pthread_get_name_np(pthread_t, char *, size_t);
 void pthread_set_name_np(pthread_t, const char *);
 int pthread_stackseg_np(pthread_t, stack_t *);
 int pthread_main_np(void);