implement robust mutexes

some of this code should be cleaned up, e.g. using macros for some of
the bit flags, masks, etc. nonetheless, the code is believed to be
working and correct at this point.
diff --git a/src/internal/pthread_impl.h b/src/internal/pthread_impl.h
index 9d56e8f..e3a9a0e 100644
--- a/src/internal/pthread_impl.h
+++ b/src/internal/pthread_impl.h
@@ -37,6 +37,11 @@
 	void **tsd;
 	pthread_attr_t attr;
 	volatile int dead;
+	struct {
+		void **head;
+		long off;
+		void *pending;
+	} robust_list;
 };
 
 #define __SU (sizeof(size_t)/sizeof(int))