sftp-server: implement limits@openssh.com extension

sftp-server: implement limits@openssh.com extension

This is a simple extension that allows the server to clearly
communicate transfer limits it is imposing so the client doesn't
have to guess, or force the user to manually tune.  This is
particularly useful when an attempt to use too large of a value
causes the server to abort the connection.

Patch from Mike Frysinger; ok dtucker@