Update to style(9) guidelines.

Update to style(9) guidelines.

- Explicitly state that the ``register'' keyword should be avoided.
- Correct example and description of preferred indentation when wrapping
  function arguments over multiple lines.
- Resolve contradictions in guidelines for ``return''.
- Explicitly state whitespace rules for commas, semicolons, ``->'' and
  ``.'' operators.
- General clarifications.

Approved-by: dillon