Correct the use of pthread_create() in several places.

Correct the use of pthread_create() in several places.

The second argument should be a pointer to pthread_attr_t,
not a numeric constant. Joinable detachstate is also the
default, we don't need to explicitly set it. Lastly, use
"NULL" for null pthread_attr_t pointer, not "0" (this is
just a cosmetic/consistency thing, but still).