hyp1f1: evaluate ratio of gammas via log(gamma)

In view of gh-2957, I propose this modification where the evaluation of the ratio of two gamma functions is done via a difference of logarithms of gamma functions. I am not completely happy with the fact, that in ``CHGM``, which takes only real arguments, the logarithm of the gamma function for complex values ``CGAMA`` is used. However, with the functions presently available in ``specfun.f``, this seems to be the most straightforward way.

While these modifications solve the problem stated in gh-2957, it does not really improve the number of test failures in ``test_mpmath.py:TestSystematic.hyp1f1`` and ``hyp1f1_complex``. So, in the long run, more work on ``hyp1f1`` will be needed.