Rework the internal lock interface

With this patch, the 'futex' syscall gets used for blocking and unblocking
of threads in the Linux-specific lock implementation.

The 'Native_thread_id' type, which was previously used in the
lock-internal 'Applicant' class to identify a thread to be woken up,
was not suitable anymore for implementing this change. With this patch,
the 'Thread_base*' type gets used instead, which also has the positive
effect of making the public 'cancelable_lock.h' header file
platform-independent.

Fixes #646.