More meta-GGA cleanup: CP now uses same interfaces tau_xc and tau_xc_spin
as PW, thus allowing more metaGGA to be used in CP as well


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11088 c92efa57-630b-4861-b058-cf58834340f0