pgcd(), chinrem_gcd(): use appropriate definition of the degree.

pgcd(), chinrem_gcd(): use appropriate definition of the degree.

Effect: fixes (rare) GCD miscalculation.

pgcd() treats polynomials Z_p[x_0, ..., x_n] as R[x_0, ..., x_{n - 1}], where
R = Z_p[x_n]. Therefore one should use correct definition of the degree
(i.e. compute it w.r.t. x_0, ..., x_{n-1}, but NOT w.r.t. x_n!).

One should use appropriate definition of degree (that is, w.r.t.  x_0, ..., x_n)
in chinrem_gcd() too.

Thanks to Aless Lasaruk for a bug report.