MFC r309460

MFC r309460

thr_set_name(): silently truncate the given name as needed

Instead of failing with ENAMETOOLONG, which is swallowed by
pthread_set_name_np() anyway, truncate the given name to MAXCOMLEN+1
bytes.  This is more likely what the user wants, and saves the
caller from truncating it before the call (which was the only
recourse).

The man page changes were not merged because thr_set_name.2
does not exist on stable/10.

Sponsored by:	Dell EMC

git-svn-id: svn://svn.freebsd.org/base/stable/10@310099 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f