selinux: Use selinux_set_mapping() to avoid hardcoded constants for policy

Previous to the introduction of selinux_set_mapping(), DBus pulled
constants generated from the system's policy at build time.  But this
means it's impossible to replace the system policy without rebuilding
userspace components.

This patch maps from arbitrary class/perm indices used by D-Bus and
the policy values and handles all the translation at runtime on
avc_has_perm() calls.

Bug: https://bugs.freedesktop.org/attachment.cgi?id=88719
Reviewed-By: Colin Walters <walters@verbum.org>
Tested-By: Colin Walters <walters@verbum.org>