Improve robot-indent so it works when c-basic-offset is 'set-from-style.

[j-oberlaender]

Hi,

I noticed that in my Emacs (24.3.1, cc-mode 5.33), the robot-indent function wouldn't work right because c-basic-offset returns the symbol 'set-from-style. I didn't find a fix anywhere that provided a proper solution to this issue, so I wrote one. The function still falls back to 4 spaces if c-basic-offset is unset, but tries its best to determine the current c-default-style's offset in case 'set-from-style is used.

If you're interested in the change, feel free to take it and run with it.