new feature: save/restore command history

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

Hello PARI-dev,

We added a new feature to GP in SVN revision 12855:
It is now possible to save the history of commands (but not the results)
automatically and to recover them in the next session.

To activate it, set in your .gprc
histfile = "~/.gp_history"

or you can activate it manually with
default(histfile,"~/.gp_history")

Of course "~/.gp_history" can be replaced by the filename you want,
subject to strftime expansion.

Cheers,
Bill.