Turn `IsUFDFamily` into a category; set `One` & `Zero` of free magma ring families

[fingolfin]

trafficstars

Resolves #5998 in so far as that the error there is fixed; and then the next error this reveals. However, the example there still will lead to an error because our polynomial rings expect a one element and a free magma doesn't have one. But it works if we use a free magma with one instead:

gap> F := FreeMagmaWithOne(2);
gap> ZF := FreeMagmaRing(Integers, F);
gap> PolynomialRing(ZF, 2);
<magma>[x_1,x_2]

I can add this as a test case but I'd like to first get some sanity checks / feedback from @ThomasBreuer and @TWiedemann