Continue trying to extract myself from the pthread_mutex_* swamp.

Continue trying to extract myself from the pthread_mutex_* swamp.

Fall back to a compromise position, which makes my mutex implementation
initialiser- and structure-compatible with LinuxThreads, and ditto the
upcoming condition var implementation.  In particular this means that
((ThreadId)0) is an invalid thread ID, so vg_threads[0] is never used,
and vg_threads[1] specially denotes the "main" thread.

Remove the scheme of having a linked list of threads waiting on
each mutex.  It is too difficult to get the right semantics for
when a signal is delivered to a thread blocked in pthread_mutex_lock().
Instead, use the old scheme of each thread stating with its .waited_on_mx
field, which mutex it is waiting for.  This makes pthread_mutex_unlock()
less efficient, but at least it all works.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@100