systemd: Use systemd functions to consider systemd availability

This is another systemd implementation clean-up.  It was found that
SELinux will block OpenVPN from checking /sys/fs/cgroups.  As OpenVPN
only checked /sys/fs/cgroups and /sys/fs/cgroups/systemd to see if
systemd was available or not, it was considered better to query
systemd directly to see whether or not to query for usernames and
passwords via systemd.

This patch has been compile tested on Fedora 19 and Fedora 21 alpha and
function tested on Fedora 19.

  v2 - Use PKG_CHECK_MODULES() + check for libsystemd before
       libystemd-daemon.  systemd >= 209 use a unified library

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1412356567-27125-1-git-send-email-openvpn.list@topphemmelig.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9072
Signed-off-by: Gert Doering <gert@greenie.muc.de>