Mediation of processes becoming a monitor

Mediation of processes becoming a monitor

When an AppArmor confined process wants to become a monitor, a check is
performed to see if eavesdropping should be allowed.

The check is based on the connection's label and the bus type.

This patch reuses the bus_apparmor_allows_eavesdropping() hook.

An example AppArmor rule that would allow a process to become a monitor
on the system bus would be:

  dbus eavesdrop bus=system,

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75113
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>