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/TightDecoder.cxx b/common/rfb/TightDecoder.cxx
index cab49fd..1bfa7a9 100644
--- a/common/rfb/TightDecoder.cxx
+++ b/common/rfb/TightDecoder.cxx
@@ -165,7 +165,7 @@
   bufptr += 1;
   buflen -= 1;
 
-  // Flush zlib streams if we are told by the server to do so.
+  // Reset zlib streams if we are told by the server to do so.
   for (int i = 0; i < 4; i++) {
     if (comp_ctl & 1) {
       zis[i].reset();
@@ -309,8 +309,8 @@
     netbuf = new rdr::U8[dataSize];
 
     zis[streamId].readBytes(netbuf, dataSize);
-    zis[streamId].reset();
 
+    zis[streamId].removeUnderlying();
     delete ms;
 
     bufptr = netbuf;