Install policy headers and include them in the refpolicy dev package.

To do this we call the 'install-headers' make target at the end of
do_install. We then add the interface 'include' directory to the
dev package leaving only the policy modules in the main policy
package. This allows projects that ship their own SELinux policy
(not in the refpolicy) to build the refpolicy headers / interface
files by using the Makefile supplied by refpolicy.

Signed-off-by: Philip Tricca <flihp@twobit.us>
Cause the SELinux config file to be staged in the sysroot by appending to sysroot_stage_all.

This is needed to build policy modules outside of refpolicy. Their
build systems need to determin the name of the policy that will be
in effect on the target host. This allows them to locate the policy
headers that will be under $sysroot/usr/share/selinux/$name/include.
Given that there *could* be more than one policy installed in the
sysroot we can't assume that the policy installed there is the only
policy to build against.
Add recipe to build example policy module.