trapping or not trapping siginter

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

Hello PARI-dev,

I found a (convoluted) way to not trap siginter:

Just use two imbricated traps:
exit()=allocatemem(default(parisize,,1))
trap(,RECOVER,trap("siginter",exit,CODE))

when Control-C is pressed while GP is evaluating code, exit()
will be called, thus by-passing RECOVER.

This is useful if you use trap() inside a long running loop.

More importantly, this imply that the internal PARI exception can be
tweaked to not siginter a specific execption with moderate changes.

Cheers,
Bill.