`random_ff'
-----------

random_ff()
     :: Random generation of an element of a finite field.

RETURN
     element of a finite field

   * Generates an element of the current base field randomly.

   * The same random generator as in `random()', `lrandom()' is used.

     [0] random_ff();
     random_ff : current_ff is not set
     return to toplevel
     [0] setmod_ff(pari(nextprime,2^40));
     1099511627791
     [1] random_ff();
     561856154357
     [2] random_ff();
     45141628299

References
     *Note `setmod_ff': setmod_ff, *Note `random': random, *Note
     `lrandom': lrandom

