minor inconsistencies related to generator names

[mezzarobba]

• I think gr_ctx_set_gen_names should work in the case of a single generator; this is not the case with univariate polynomials, nor for number fields.
• Neither gr_ctx_set_gen_name nor gr_ctx_set_gen_names appears to have an effect for finite fields (gr/fq*).
• I suppose gr Gaussian integers, complex floating-point numbers, and complex balls could also support renaming i, though this is far from essential.
• (I didn't test generic power series, since they changed this morning!)

[fredrik-johansson]

Fixing these issues in #1878 except for

I suppose gr Gaussian integers, complex floating-point numbers, and complex balls could also support renaming i, though this is far from essential.

My first thought was why on earth would do do that? But on second thought there might be a legitimate case for allowing switching between i or I or j, say. I don't think I'm going to implement this but if someone really cares I guess they can submit a PR.