MAINT: GLM

Addresses (or partially addresses) several issues with GLM:

* Allow use of maxiter==0 to evaluate the likelihood at a given point (closes #1728).  Previously maxiter==0 was doing one iteration (should be zero iterations).  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)
the loop doesn't stop early anymore, needs
```
if converged:
    break
```
and setting an indicator if maxiter has been reached, e.g. an `else` to the `for` for `converged=False`

```
for iteration in range(maxiter)
    ....
    if (...): break
else:
    <this is the part that we only reach if the loop was not terminated with a break, ie with maxiter>
    converged = False
```