Rename m-mutex header into m-thread

as it is not just about mutex but about thread functions
just like the header pthread or thread.h

tests/test-mishared.c

@@ -25,7 +25,7 @@
 
 #include "m-i-shared.h"
 
-#include "m-mutex.h"
+#include "m-thread.h"
 #include "coverage.h"
 
 typedef struct ishared_s {