issymetric.m; Overhaul function and expand to check skew-symmetry (bug #53556). * issymmetric.m: Rewrite docstring to discuss second argument SKEWOPT which can be "skew" or "nonskew". Add background material on what symmetric and skew-symmetric mean. Redo input parsing. Use any() rather than nnz() for a potential savings of 0-99% on execution time depending on characteristics of matrix. Add more BIST tests.

gnulib = http://hg-new.octave.org/gnulib