brp-mangle-shebangs: make it possible to use generated exclusion lists
    
    This patch adds two additional rpm macros, __brp_mangle_shebangs_exclude_file
    and __brp_mangle_shebangs_exclude_from_file, to specify files from which
    to read the extended regexps used for excluding shebangs and target
    files.
    
    Additionally, this adds documentation in the macros file and
    --help/--usage/-?/-h to brp-mangle-shebangs, so that it's possible to
    actually discover what the intended behavior is without reading the
    script itself.
    
    Signed-off-by: Peter Jones <pjones@redhat.com>