systemd: Add more security feature for systemd units

systemd: Add more security feature for systemd units

ProtectSystem=true mounts the /usr and /boot directories read-only.

ProtectHome=true makes the directories /home, /root and /run/user
inaccessible and empty for the process.

See systemd.exec(5) [0] for details.

v2: Replace ProtectSystem=strict with ProtectSystem=true. Some
    configurations may want to write to /etc or the like.

[0] https://www.freedesktop.org/software/systemd/man/systemd.exec.html

Signed-off-by: Christian Hesse <mail@eworm.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20161227221832.610-1-list@eworm.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13743.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 76096c605fcac4815674b6ae76ac1f31f03a8186)