[PATCH] vi: add expandtab option

[Ron Yorston]

Some thoughts on this:

- When a tab is being replaced during insertion sufficient spaces
  should be added to reach the next tabstop, this may not be equal
  to the tabstop value.

- The spaces inserted aren't removed by undo.

- Processing of tabs doesn't happen when text is being replaced
  rather than inserted.

- The expandtab option isn't added to the status message produced
  by ':set all'.  (Actually, it occurs to me that this could have
  been avoided if the message was generated by iterating through
  the options.  I wasn't able to achieve any reduction in bloat
  when I tried this, though.  Maybe if we had more options...)

In addition, as currently implemented, ctrl-D doesn't work very well
with spaces in the indent.

The attached patches are my attempt at addressing these issues.

I've haven't signed off on them because I'd like to get a signoff
from Peter on the first and some feedback on whether they'd be
acceptable, given the 310 bytes of bloat they add.  Also, some
testing would be nice.

Ron