openssh: enable PAM at runtime based on DISTRO_FEATURES

openssh: enable PAM at runtime based on DISTRO_FEATURES

Everything is setup to use PAM except for the server config. If 'pam' is
in DISTRO_FEATURES the configs will be changed to enable it.

Syslog will now show:

	Feb 19 09:28:36 beast sshd[2980]: pam_unix(sshd:session): session opened for user koen by (uid=0)

And more importantly:

	koen@beast:~$ loginctl
	   SESSION        UID USER             SEAT
	        c1       1000 koen             seat0
	        c3       1000 koen             seat0
	       c13       1000 koen

	3 sessions listed.

Systemd now registers the session properly so it won't kill things like 'screen'
and 'tmux' when disconnecting the ssh session.

Signed-off-by: Koen Kooi <koen.kooi@linaro.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>