Let exponentiation of polynomial using FLINT nmod type be interrupted

[jpflori]

sage: p = next_prime(2**30)
sage: k = GF(p)
sage: x = polygen(k)
sage: x**p
CTRL+C....

CC: @vbraun @jdemeyer

Component: finite rings

Author: Jean-Pierre Flori

Branch/Commit: 0706674

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/17470

[jpflori]

Commit: ef28e27

[jpflori]

Author: Jean-Pierre Flori

[jpflori]

Branch: u/jpflori/ticket/17470

[jpflori]

New commits:

0aed331	Make modular exponentiation of polynomial using FLINT interruptable.
ef28e27	Better fix for exponentiation of polys using FLINT nmod and test.

[jpflori]

Description changed:

--- 
+++ 
@@ -1,6 +1,7 @@
 
 ```
-sage: k = GF(next_prime(2**30))
+sage: p = next_prime(2**30)
+sage: k = GF(p)
 sage: x = polygen(k)
 sage: x**p
 CTRL+C....

[jdemeyer]

comment:2

cdefs.pxi should be interrupt.pxi

[jdemeyer]

comment:3

I know it's not the topic of this ticket, but why is this code not implemented as direct calls to either nmod_poly_pow() or nmod_poly_powmod_mpz_binexp()?

[jdemeyer]

comment:4

Also, celement_pow doesn't seem to be documented anywhere...

I do give positive_review to the addition of sig_on() and sig_off() and the doctest changes.