dropbear: add options SSHKeepAlive and IdleTimeout.

Without timeout mechanism, if ssh client disconnected without sending
FIN or RST, forked dropbear servers would hang there for
KEX_RETRY_TIMEOUT seconds (8 hours).

TCP keepalive is not implemented in dropbear yet, thus the name
SSHKeepAlive.

300 seconds in this patch is selected from the default value of
ServerAliveInterval for Debian ssh client (See man ssh_config).

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

SVN-Revision: 40299