NetBSD's pthread_setname_np() prototype is incompatible with the rest
of the world, and it requires to pass the string argument as a
non-const pointer. Instead of working around this misdesign, I hereby
disable the feature on NetBSD.
Add macro HAVE_THREAD_NAME which is set when any method to set the
thread name is available. Use that macro in FormatThreadName()
instead of just checking for HAVE_PTHREAD_SETNAME_NP.
