Add SELinux support to the build system

X-Gentoo-Bug: 516956
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=516956
tmpfiles: set the proper SELinux context

Restore the label on the created file / dir based on the policy
fcontexts.

X-Gentoo-Bug: 516956
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=516956
devfs: fix SELinux contexts

SELinux contexts in /dev need to be fixed after it is mounted

X-Gentoo-Bug: 516956
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=516956
checkpath: restore the SELinux context

X-Gentoo-Bug: 516956
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=516956