MAINT: GLM

Addresses (or partially addresses) a number of issues with GLM:

* Use maxiter==0 to evaluate the likelihood at a given point (closes #1728).  Previously maxiter==0 was still doing one iteration.  Added a test for this use case.

* Rename eta -> lin_pred and Y -> endog in many places.  Partially closes #1685, but I didn't do anything with mu (naming issues with links, e.g. fitted and predict still stand).

* Rename confusing keyword argument fittedvalues in the negative binomial log likelihood to  lin_pred (which is what it really is).

* Avoid making unneeded copy of offset (second part of #1720)