Clear up ZlibInStream::reset() behaviour

It previously only did a reset of the ZlibInStream object, not the
underlying zlib stream. It also had the side effect of flushing
the underlying stream and disassociating from it.

Clear things up by changing the naming, and introducing a proper
reset function (which is needed by the Tight decoder).
diff --git a/common/rfb/zrleDecode.h b/common/rfb/zrleDecode.h
index 2566171..07d6795 100644
--- a/common/rfb/zrleDecode.h
+++ b/common/rfb/zrleDecode.h
@@ -177,7 +177,7 @@
     }
   }
 
-  zis->reset();
+  zis->removeUnderlying();
 }
 
 #undef ZRLE_DECODE