Create a print_usage() function for usage printing

The source code text is unchanged, modulo whitespace and s/./->/g.
Reformat usage to (mostly) fit in 80 columns

This uses the typical GNU-style spacing (e.g., two leading spaces on
option lines) and removes brackets around <FILE> etc, as the capitals
used distinguishes them without using up characters.
Minor rewording of usage to fit in 80 columns