Correctly restore NTL contexts for finite field extensions.
Better fix for NTL contexts.
Add test for correct restoration of NTL contexts.
Remove previous changes and fix typo.
Merge remote-tracking branch 'trac/develop' into ticket/9524

Conflicts:
	src/sage/rings/polynomial/polynomial_zz_pex.pyx
Trac #9524: Nasty bug with polynomial arithmetic and NTL contexts

I just ran into the following nasty bug:

{{{
sage: polygen(GF(49, 'a')) ; polygen(GF(9, 'a'))
x
x
sage: x = polygen(GF(49, 'a'))
sage: -x
2*x
sage: x + 0
x
sage: -x
6*x
}}}

This is still present in sage-4.5.

We have to ensure two things before playing with NTL finite field
elements:
* restore the ZZ_p context (characteristic);
* restore the ZZ_pE context (defining polynomial).
Restoring the latter does not automatically restore the former.

URL: http://trac.sagemath.org/9524
Reported by: craigcitro
Ticket author(s): Jean-Pierre Flori
Reviewer(s): Volker Braun