Initial support for NSS plugin skeleton
    
    Name Service Switch is a glibc feature responsible for many
    things. Translating domain names into IP addresses and vice versa
    is just one of them. However, currently it's the only
    functionality that this commit is tickling. Well, in this commit
    the plugin skeleton is introduced. Implementation to come in next
    patches.
    Because of the future testing, where the implementation is to be
    linked with a test, this needs to go into static library. Linking
    a program with an .so statically is not portable. Therefore a
    dummy libnss_libvirt_impl library is being introduced too.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>