Changed the spec for mutex::unlock() to say that it is illegal for a thread to

unlock a dlib::mutex it doesn't own.  This is technically a non-backwards
compatible change to the API but it has always been implemented in a way that
didn't allow this kind of double unlock() on non-Windows platforms.
Additionally, the recent change to the dlib::mutex implementation on windows
also no longer allows this kind of double unlock.

dlib/threads/threads_kernel_abstract.h

@@ -175,11 +175,10 @@ namespace dlib
         void unlock (
         ) const;
         /*!
+            requires
+                - the thread calling unlock() already has a lock on *this
             ensures
-                - if (*this is currently locked and owned by the thread calling unlock) then
-                    - #*this is unlocked (i.e. other threads may now lock this object)
-                - else
-                    - the call to unlock() has no effect
+                - #*this is unlocked (i.e. other threads may now lock this object)
         !*/