use paxctl-ng to support XATTR_PAX.

Gentoo is moving towards pax filesystem marking (XATTR_PAX) instead of the Elf Header (PT_PAX) (1). However paxctl does only add the PT_PAX marking. The Problem is that ebuilds should use the pax eclass (2) for marking which is then invoking the right tools, however because we have to modify the makefile itself I don't know if is possible to invoke this function from outside the ebuild. If it is this method should be preferred over this pull request. If it is not (or very difficult) this pull request should fix the issue as paxctl-ng sets both markings XATTR_PAX and PT_PAX. 

Btw: for this reason gentoo-hardened should have an executable pax-mark which does exactly what the pax-mark function of the pax-eclass does. But I don't think gentoo has something like that currently.

(1) http://www.gentoo.org/proj/en/hardened/pax-migrate-xattr.xml
(2) http://devmanual.gentoo.org/eclass-reference/pax-utils.eclass/index.html