use-package: use defun as lisp-indent-function

When `use-package` is called with only one keyword it is useful to
write:

```
(use-package foo :init
  (progn
    ... long lines ...))
```

instead of

```
(use-package foo
  :init (progn
  	  ... *too* long lines ...))
```

or

```
(use-package foo
  :init
  (progn
    ... long lines ...))
```

Even when there are multiple keywords or when one never wants to format
the calls to `use-package` as in the first example the use of `defun`
does not really pose a problem.