Initialize AppArmor mediation

Initialize AppArmor mediation

When starting dbus-daemon, autodetect AppArmor kernel support and use
the results from parsing the busconfig to determine if mediation should
be enabled.

In the busconfig, "enabled" means that kernel support is autodetected
and, if available, AppArmor mediation occurs in dbus-daemon. In
"enabled" mode, if kernel support is not detected, mediation is
disabled. "disabled" means that mediation does not occur. "required"
means that kernel support must be detected for dbus-daemon to start.

Additionally, when libaudit support is built into dbus-daemon, the
AppArmor initialization routines set up the audit connection.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75113
Signed-off-by: John Johansen <john.johansen@canonical.com>
[tyhicks: Honor enforcement modes and detect AppArmor dbus rule support]
[tyhicks: fix unreachable return when AppArmor support is built]
[tyhicks: make bus_apparmor_full_init() able to raise a DBusError]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
[smcv: _bus_apparmor_aa_supports_dbus: document necessary kernel API guarantee]
[smcv: bus_apparmor_pre_init: distinguish between OOM and AppArmor not enabled]
[smcv: document why we open() and not just stat()]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>