Rename native_mutex to fast_mutex throughout.

Rename native_mutex to fast_mutex throughout.
Rename pthread_key::save_key_to_buffer to
pthread_key::_fixup_before_fork throughout.
Rename pthread_key::recreate_key_from_buffer to
pthread_key::_fixup_after_fork throughout.

* thread.cc (native_mutex::init): Remove.
(native_mutex::lock): Ditto.
(native_mutex::unlock): Ditto.
(pthread::push_cleanup_handler): InterlockedExchangePointer
is not needed here.
(pthread_rwlock::pthread_rwlock): Initialize readers list mutex.
(pthread_rwlock::add_reader): Add reader via List_insert.
(pthread_rwlock::lookup_reader): Lock list while walking through.
(pthread_cond::init): Locking the init mutex is now void.
(pthread_rwlock::init): Ditto.
(pthread_mutex::init): Ditto.
* thread.h: Include security.h.
(fast_mutex): New class. Replacement for native_mutex.
(List_insert): New template function.
(List_remove): Ditto.
(List::List): Initialize synchronising mutex.
(List::fixup_after_fork): New method.
(List::insert): Add node via List_insert.
(List::remove): Remove node via List_remove.
(List::pop): Remove.
(List::for_each): Lock list while walking through.
(List::mx_init): New method.
(pthread_mutex::fixup_after_fork): Fixup mutex list after fork.
(pthread::fixup_after_fork): Ditto.
(pthread_conds::fixup_after_fork): Ditto.
(pthread_rwlock::fixup_after_fork): Ditto.
(semaphore::fixup_after_fork): Ditto.
(pthread_rwlock::readers_mx): New member.