Add support for fixed-length records
Refactor record-separator code to use a window
Move common commands to the end
Update usage information
No need to pass program name on usage calls
Add support for operations on windows

Windows are defined by time or record numbers

While at it perform extensive refactoring of debug and test
infrastructure.
Rename teebuff tests into tee

This completes the renaming of teebuff into sgsh-tee.
Refactor long main function