Fixes for pthread thread local storage.

Fixes for pthread thread local storage.

The tls_name_key variable is passed to tls_name_get(), and the first
time this happens it isn't initialized. tls_name_get() then passes it
on to tls_name_alloc() which passes it on to pthread_setspecific()
leading to undefined behavior.

None of this is actually necessary at all because there is only one
such variable per thread local variable, so it doesn't need to passed
as a parameter at all.

All of this was pointed out by Tor Lillqvist on the cairo mailing
list.