Convert tabs to 8 spaces in editable files.

[efiring]

The C source uses a mix of tabs and spaces, which wreaks havoc
when editing with an editor configured for 4-space tabs.
Therefore, to preserve the present visual structure under
future editing, all tab characters in user-editable files
are replaced with 8 spaces; hard tab characters (and trailing
whitespace) should not be used in any future edits.

[efiring]

I recognize that there is some distaste for massive style changes like this, but I consider this one to be essential for maintaining my sanity if I am to continue working with this codebase. Note that the aim is to preserve the visual appearance of the existing code, regardless of the editor that is used to look at it or modify it; this is a change in the way the present style is encoded, not a change in the style itself.