Use proper CHAN_TCP_PACKET_DEFAULT for agent forwarding when HPN disabled.

The use of CHAN_TCP_WINDOW_DEFAULT here was fixed in upstream OpenSSH
in CVS 1.4810, git 5baa170d771de9e95cf30b4c469ece684244cf3e:

• dtucker@cvs.openbsd.org 2007/12/28 22:34:47 [clientloop.c] Use the correct packet maximum sizes for remote port and agent forwarding. Prevents the server from killing the connection if too much data is queued and an excessively large packet gets sent. bz #1360, ok djm@.

The change was lost due to the the way the original upstream HPN patch
modified this code. It was re-adding the original OpenSSH code and never
was properly fixed to use the new value.

MFC after: 2 weeks