Added two small files from libxc. Compile with -D__LIBXC, link with libxc:
-L/dir/where/libxc/is -lxcf90 -lxc
User guides updated with a reshuffling of names: contributors to all of QE
(in particular, most functionals) are listed in the user guide of QE and
not in the one of PW. make.depend updated

This is just to avoid that the work done is lost. It should be a first step 
towards better organization of XC functionals. We should seriously consider
one of the following options:
1. move to libxc, getting rid of all duplicated functionals, keeping only
   those that are not available in libxc;
2. add the possibility to use any functional from libxc (currently only a few
   can be used). This may require serious restructuring work in our XC
   functional (il-)logic, that has become clumsy, obscure and redundant


git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11075 c92efa57-630b-4861-b058-cf58834340f0