#4 Use actual program name in usage() statement.

[Frederick Bruckman]

Let the usage statement use the actual name by which
the program was invoked (set globally to argv[0], in
main()).

The NetBSD package system, for instance, can't install
this program as "par", because it would conflict with
the venerable paragraph formatting utility by the same
name. It looks dumb, then, when the usage statement
doesn't match.

[Frederick Bruckman]

Use agv[0] in usage() statement.