pscp, psftp: use a bufchain in ssh_scp_recv.

The ad-hoc code that received data from the SCP or SFTP server
predated even not-very-modern conveniences such as bufchain, and was
quite horrible and cumbersome.

Particularly nasty was the part where ssh_scp_recv set a _global_
pointer variable to the buffer it was in the middle of writing to, and
then recursed and expected a callback to use that pointer. That caused
clang-analyzer to grumble at me, in a particular case where the output
buffer was in the ultimate caller's stack frame; even though I'm
confident the code _worked_, I can't blame clang for being unhappy!

So now we do things the modern and much simpler way: the callback when
data comes in just puts it on a bufchain, and the top-level
ssh_scp_recv repeatedly waits until data arrives in the bufchain and
then copies it to the output buffer.

misc.c

@@ -813,6 +813,15 @@ bool bufchain_try_fetch_consume(bufchain *ch, void *data, int len)
     }
 }
 
+int bufchain_fetch_consume_up_to(bufchain *ch, void *data, int len)
+{
+    if (len > ch->buffersize)
+        len = ch->buffersize;
+    if (len)
+        bufchain_fetch_consume(ch, data, len);
+    return len;
+}
+
 /* ----------------------------------------------------------------------
  * Sanitise terminal output that we have reason not to trust, e.g.
  * because it appears in the login banner or password prompt from a