independently adapt indent-offset and indent-tabs-mode

Hi,

I suggest three modular changes that are rather self-explanatory.

The motivation to allow for separate adaption of indent-offset and indent-tabs-mode is the following:
If indent-offset is not changed for any reason, indent-tabs-mode could be change anyway, couldn't it?

To also support "unsupported" major modes, I suggest to introduce a "default" fallback, which simply adapts
standard-indent.

Please decide on the general usefulness of the contributions.

Best, Robert