normal.cpp, normal.h: Fix Yun's algorithm (there was a mistake in Geddes et

normal.cpp, normal.h: Fix Yun's algorithm (there was a mistake in Geddes et
al.) for square-free factorization.  Also change the syntax to allow for
square-free factoring in a recursive way in Q[X].  This is an example how it
works in Z[x,y,z] in ginsh:
> ?sqrfree
sqrfree(expression [, symbol-list]) - square-free factorization of a polynomial
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3));
(x+y)*(-x-y-z)^2*(y^3+3*y*z^2-3*y^2*z-z^3)
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[y]);
(x+y)*(y-z)^3*(x+y+z)^2
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[z]);
(x+y)*(y-z)^3*(x+y+z)^2
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[x]);
(x+y)*(-x-y-z)^2*(y^3+3*y*z^2-3*y^2*z-z^3)
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[x,y]);
(x+y)*(-x-y-z)^2*(y^3+3*y*z^2-3*y^2*z-z^3)
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[x,y,z]);
(x+y)*(-x-y-z)^2*(y^3+3*y*z^2-3*y^2*z-z^3)
> sqrfree(expand((x+y)*(x+y+z)^2*(y-z)^3),[v]);
3*x*y^5+3*x^2*y^4+x^3*y^3+y^6-y*z^5-2*y^4*z^2+y^2*z^4-y^5*z+2*y^3*z^3-3*x^3*y^2*z+3*x^3*y*z^2-5*x*y^4*z-x*y*z^4+3*x^2*y*z^3+6*x*y^2*z^3+3*x^2*y^2*z^2-2*x*y^3*z^2-7*x^2*y^3*z-x^3*z^3-2*x^2*z^4-x*z^5