implement CopyFile without using FileFd::Size()

Pipes and such have no good Size value, but we still want to copy from
it maybe and we don't really need size as we can just as well read as
long as we get data out of a file to copy it.

Git-Dch: Ignore

apt-pkg/contrib/fileutl.cc

@@ -158,24 +158,18 @@ bool CopyFile(FileFd &From,FileFd &To)
    if (From.IsOpen() == false || To.IsOpen() == false ||
 	 From.Failed() == true || To.Failed() == true)
       return false;
-   
+
    // Buffered copy between fds
    std::unique_ptr<unsigned char[]> Buf(new unsigned char[64000]);
-   unsigned long long Size = From.Size();
-   while (Size != 0)
-   {
-      unsigned long long ToRead = Size;
-      if (Size > 64000)
-	 ToRead = 64000;
-      
-      if (From.Read(Buf.get(),ToRead) == false ||
+   constexpr unsigned long long BufSize = sizeof(Buf.get())/sizeof(Buf.get()[0]);
+   unsigned long long ToRead = 0;
+   do {
+      if (From.Read(Buf.get(),BufSize, &ToRead) == false ||
 	  To.Write(Buf.get(),ToRead) == false)
 	 return false;
-      
-      Size -= ToRead;
-   }
+   } while (ToRead != 0);
 
-   return true;   
+   return true;
 }
 									/*}}}*/
 // GetLock - Gets a lock file						/*{{{*/