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

lib/libmthread/shlib_version

@@ -0,0 +1,2 @@
+major=0
+minor=0