Add an ftruncate call paired with fallocate to play safe with some FSes (namely, BtrFS fallocate sets file size to allocated size, i.e. multiple of block size)

src/libutil/archive.hh

@@ -64,6 +64,7 @@ struct ParseSink
     virtual void isExecutable() { };
     virtual void preallocateContents(unsigned long long size) { };
     virtual void receiveContents(unsigned char * data, unsigned int len) { };
+    virtual void finalizeContents(unsigned long long size) { };
 
     virtual void createSymlink(const Path & path, const string & target) { };
 };