Pull the pthread_key_t functions out of pthread.c.

Pull the pthread_key_t functions out of pthread.c.

This was originally motivated by noticing that we were setting the
wrong bits for the well-known tls entries. That was a harmless bug
because none of the well-known tls entries has a destructor, but
it's best not to leave land mines lying around.

Also add some missing POSIX constants, a new test, and fix
pthread_key_create's return value when we hit the limit.

Change-Id: Ife26ea2f4b40865308e8410ec803b20bcc3e0ed1