new debugging facility

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

Dear PARI-dev,

I have commited to PARI svn repository a set of patch which allow
better error reporting and debugging.
1) Now errors are displayed this way:

? f(n)=sum(p=2,n,quadclassunit(-p)[1]);
? f(100)
  ***   at: f(100)
            ^------

  ***   at: sum(p=2,n,quadclassunit(-p)[1]
                      ^--------------------
  *** quadclassunit: discriminant not congruent to 0,1 mod 4 in Buchquad.

2) trap() debugging facility is restored to 2.3 level:
? trap();
? f(100)
  ***   at: f(100)
            ^------

  ***   at: sum(p=2,n,quadclassunit(-p)[1]
                      ^--------------------
  *** quadclassunit: discriminant not congruent to 0,1 mod 4 in Buchquad.
  ***   Break loop (type 'break' or Control-d to go back to GP)
break> p
2
break> n
100

(before that change, it would return 'n' and 'p'.)

Of course this is only a start. We would like to hear about your
suggestions.

Cheers,
Bill.