Add fixwhite(1).

Add fixwhite(1).

This small utility can be used to `sanitize' the whitespace in source
code. It does the following things:

Global:
- Remove empty lines at the beginning and the end of a file.
- Merge successive empty lines into a single empty line.

Per-line:
- Remove trailing whitespace.
- Merge spaces preceeding tabs into the tabs.

It operated on stdin/stdout. This means that if you use vi(1), you can
just run :%!fixwhite to reorganize the file.