* thread.cc: Partial refactoring of pthread_key destructor
handling. Loosely based on Thomas Pfaff's work.
(pthread_key_destructor_list::Insert): Remove.
(pthread_key_destructor_list::Pop): Remove.
(pthread_key_destructor_list::IterateNull): Call the key's
run_destructor method.
(pthread_key::pthread_key): Initialize new member.
(pthread_key::get): Mark as const for correctness.
(pthread_key::run_destructor): Implement.
* thread.h (pthread_key::get): Mark as const for correctness.
(pthread_key::run_destructor): Declare.
(List): New template class that implements a generic list.
(pthread_key_destructor_list): Inherit from List, and remove
now duplicate functions.
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