Branch bill-mt for parallel pari updated

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

Dear PARI developers,

I have updated the GIT branch bill-mt for parallel pari again.
This branch support
- POSIX threads: do
- MPI (for clusters)

The documentation is available at
<http://pari.math.u-bordeaux.fr/~bill/parallel.pdf>
and hasve been improved.

The following parallel GP functions are available:
parapply, pareval, parsum, parvector, parfor and parforprime.

When using POSIX threads, this version allows to interrupt parallel tasks
(for example when the user do ^C^D or when an error occurs in another task).
This is difficult to get right, so please test!

Cheers,
Bill.