Move DEBUG macro body to debug_fn
    
    Move DEBUG macro body to the debug_fn function, adding "function"
    argument to the latter.
    Rename "debug_fn" in sssd_krb5_locator_plugin.c to "plugin_debug_fn" to
    remove conflict with the sssd debug_fn.
    Replace DEBUG_MSG macro usage with debug_fn function usage.
    Remove DEBUG_MSG macro along with tests.
    
    The above makes the total size of binaries drop by 20% for the standard
    Fedora build and by 44% for a build configured according to Debian
    packaging script.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Stephen Gallagher <sgallagh@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    (cherry picked from commit fb0332565892bc10998ca98b567d4dde2213844d)