Fix file checks when --chroot is being used

Commit 0f2bc0dd92f43c9 started to introduce some file sanity
checking before OpenVPN started to avoid harder to explain issues
due to missing files or directories later on.  But that commit
did not consider --chroot at all.  Which would basically cause
OpenVPN to complain on non-missing files, because it would not
consider that the files where inside a chroot.

This patch is based on the thoughts in a patch by Josh Cepek [1],
but trying to simplify it at bit.

[1] <http://thread.gmane.org/gmane.network.openvpn.devel/7873>,
    (Message-ID: l142b7$15v$1@ger.gmane.org)

[v2 - Simplify the changes in check_cmd_access(), let the chroot
      tackling happen only in check_file_access_chroot() only]

Trac-ticket: 330
Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1385382778-4723-1-git-send-email-dazo@users.sourceforge.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8060
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b77bffe8186647c6fd1f2f76aac41fd45719edb8)