Experimental pthread compatibility library

This patch adds pthread compatibility by using libmthread.

To use this with a program using pthreads, you have to replace
  #include <pthread>
with
  #define _MTHREADIFY_PTHREADS
  #include <minix/mthreads>

This also changes the initialization function to be a constructor, which
is implicitly called before the call to main. This allows for
conformance with pthreads, while not paying a high price by checking on
each mthread_* call whether the library has been initialized or not.

As mthread_init is now a constructor, it also has been set as static, and
relevent calls removed from programs using it.

Change-Id: I2aa375db557958d2bee9a70d285aabb990c88f00

test/test59.c

@@ -1116,7 +1116,6 @@ int main(void)
   once = MTHREAD_ONCE_INIT;
 
   start(59);
-  mthread_init(); 
   test_scheduling();
   test_mutex();
   test_event();