Fix spurious EOF in agent forwarding!

Commit 6a8b9d381, which created the Channel vtable and moved the agent
forwarding implementation of it out into agentf.c, managed to set the
rcvd_eof flag to TRUE in agentf_new(), meaning that we behave exactly
as if the first agent request was followed by an incoming EOF.

agentf.c

@@ -164,7 +164,7 @@ Channel *agentf_new(SshChannel *c)
     af->c = c;
     af->chan.vt = &agentf_channelvt;
     af->chan.initial_fixed_window_size = 0;
-    af->rcvd_eof = TRUE;
+    af->rcvd_eof = FALSE;
     bufchain_init(&af->inbuffer);
     af->pending = NULL;
     af->input_wanted = TRUE;