pthread: Even easier thread-specific keys

`pthread_key_t` does not have to be an integer.

Keys don't have a usage counter. `pthread_key_delete()` deletes a key
and removes all the thread-specific values.

Speed-up frequently used keys by moving the values to the front of the
list.
pthread: move TSS implementation into its own file
pthread: link in pthread_tls.o only if used
Merge pull request #4 from Kijewski/add_pthread_simple_tls-PR

looks good