Write a version for pthread_get_name_np

[Konstantin Belousov]

On Mon, Aug 13, 2018 at 10:16:30AM +0200, Willem Jan Withagen wrote:
> I guess that'll work.
> 
> Although I was following the procstat way and when there was no name set 
> we get a '-'. But if Konstatin is going to write it, it is his prerogative.
> And I would expect it to be equivalent partner of thr_{set,get}_name as 
> well. But that could be added later in the man page.
There is no thr_get_name(2).

The implementation is available at https://reviews.freebsd.org/D16702.
I do not like void return types for both pthread_get_name_np() and
pthread_set_name_np().