Function: ffgen
Section: number_theoretical
C-Name: ffgen
Prototype: GDn
Help: ffgen(P,{v}): return the generator g=X mod P(X) of the finite field
 defined by the polynomial P(X). If v is given, the variable name is used to
 display g, else the variable of the polynomial P is used.
Doc: return the generator $g=X \pmod{P(X)}$ of the
 finite field defined by the polynomial $P$ (which must have \typ{INTMOD}
 coefficients). If \kbd{v} is given, the variable name is used to display $g$,
 else the variable of the polynomial $P$ is used.
