provide signbit mapper for real values * mappers.cc (Fsignbit): New function and tests. * ov-base.cc (octave_base_value::get_umap_name): Include signbit in the list. * ov-base.h (octave_base_value::unary_mapper_t): Include umap_xsignbit in the list. * ov-float.cc (octave_float_scalar::map): Handle xsignbit. * ov-scalar.cc (octave_scalar::map): Handle xsignbit. * ov-re-mat.cc (octave_matrix::map): Handle xsignbit. * ov-flt-re-mat.cc (octave_float_matrix::map): Handle xsignbit. * lo-mappers.h (xsignbit): New template function. * arith.txi: Document signbit.