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!)