Extract MakeUnique into utilmemory.h

And use it to reduce chainparamsbase's direct reliance on util.h to
only args handling.

utilmemory.h can be replaced with <memory> once we move to C++14.

src/chainparamsbase.cpp

@@ -7,6 +7,7 @@
 
 #include <tinyformat.h>
 #include <util.h>
+#include <utilmemory.h>
 
 #include <assert.h>