Enable ssh.socket by default and add a systemd generator to configure ListenStream= based on /etc/sshd_config entries

In Ubuntu, we have been using socket activated sshd by default for a few releases. This PR is an attempt to do the same in Debian and re-align the Debian and Ubuntu packages a bit.

First, this PR includes bug fixes to the existing systemd-socket-activation.patch, namely surrounding sshd's re-execution behavior. Second, a new patch adds a new binary, sshd-socket-generator, which is a systemd generator responsible for reading configured Port and ListenAddress options for sshd, and generating a drop-in configuration ($systemd_gen_dir/ssh.socket.d/addresses.conf) with corresponding ListenStream= overrides. Next, there are several changes to the packaging to make this setup the new default. Finally, a couple of dep8 tests are added.