Support the NetBSD version of pthread_setname_np

Support the NetBSD version of pthread_setname_np

On NetBSD, pthread_setname_np takes a printf-style format string plus
one argument:
https://netbsd.gw.com/cgi-bin/man-cgi?pthread_setname_np++NetBSD-current

This patch makes thread-pool.c handle that.

gdbsupport/ChangeLog:

2020-01-24  Christian Biesinger  <cbiesinger@google.com>

	* thread-pool.c (set_thread_name): Add an overload for the NetBSD
	version of pthread_setname_np.

Change-Id: I61e664a813eaa7f52b6811b1a43e08ac3082d8ef