nfsplitting for reducible polynomials

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear PARI/GP developers,

I have updated nfsplitting to handle reducible polynomials.
For example:

? nfsplitting((x^2+1)*(x^2+2))
%1 = x^4+6*x^2+1
? S = nfsplitting(x^6-8) // reducible
%2 = x^4+2*x^2+4
? lift(nfroots(subst(S,x,a),x^6-8))
%3 = [-a,a,-1/2*a^3-a,-1/2*a^3,1/2*a^3,1/2*a^3+a]

Cheers,
Bill.