Handle locked pages more robustly (Fixes issue #1462)

Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
those functions are used naively. In this commit a class "LockedPageManager" is added
that simulates stacking memory locks by keeping a counter per page.

src/util.cpp

@@ -86,6 +86,8 @@ void locking_callback(int mode, int i, const char* file, int line)
     }
 }
 
+LockedPageManager LockedPageManager::instance;
+
 // Init
 class CInit
 {