G_numeric: fix numeric evaluation with trailing zeros and y != 1.

Make sure G_numeric makes use of scaling relation (G_do_hoelder) only if
there are no traling zeros. Use G_do_trafo otherwise (which can handle
trailing zeros).
G_do_hoelder: fix the transformation of the imaginary part.

The G_do_hoelder function transforms the arguments as z -> 1-z.
If z has an imaginary part, the sign of the imagniary part transforms
as s -> -s.
In the special case where z is real and > 1, the transformed value is
real and < 0. For (transformed) real negative values of z the sign of
the imaginary part is not relevant (there are no branch cuts). The code
requires that in this case the sign takes the default value +1.
This is now fixed.