* thread.cc (pthread_rwlock::add_reader): Perform new operation here and return

* thread.cc (pthread_rwlock::add_reader): Perform new operation here and return
pointer to allocated RWLOCK_READER structure.
(pthread_rwlock::rdlock): Reorganize to reflect new add_reader functionality.
(pthread_rwlock::tryrdlock): Ditto.  Remove unneeded call to lookup_reader().
* thread.h (pthread_rwlock::RWLOCK_READER::RWLOCK_READER): New constructor.
(pthread_rwlock::add_reader): Reflect new functionality.