znlog improvement

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Hello PARI-dev,

I have commited a patch that implement the linear sieve algorithm for discrete
logarithm in znlog, which is much faster (subexponentation). While the
implementation is not optimal, now znlog can compute logarithms modulo a
150 bits prime.

I also have plans for improving fflog.

Cheers,
Bill.