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

bench/bench-mlib.c

@@ -14,7 +14,7 @@
 #include "m-mempool.h"
 #include "m-string.h"
 #include "m-buffer.h"
-#include "m-mutex.h"
+#include "m-thread.h"
 #include "m-deque.h"
 #include "m-concurrent.h"
 #include "m-tuple.h"

bench/common.h

@@ -31,7 +31,7 @@
 
 // Needed for M_THREAD_ATTR
 #if defined(MULTI_THREAD_MEASURE)
-#include "m-mutex.h"
+#include "m-thread.h"
 #endif
 
 #if defined(_WIN32)