(indent-line-function): Change default to indent-relative.

(indent-line-function): Change default to indent-relative.
(tab-always-indent): Add an `always' setting.
(indent-according-to-mode): Handle `indent-relative' and
`indent-relative-maybe' specially.
(indent-for-tab-command): Rename `prefix-arg' to `arg'.
Handle the `always; case for `tab-always-indent'.
Don't call indent-according-to-mode for indent-relative' and
`indent-relative-maybe'.
(insert-tab): Rename `prefix-arg' to `arg'.
(indent-region): Indent the first line as well.
(indent-relative): Don't mark the buffer modified if the indentation
is unchanged.