rm: zero args versus -f

[mknos]

• NetBSD, OpenBSD and GNU versions do exit(0) for "rm -f" with no file arguments
• Standards document says -f flag should not modify the exit status for nonexistent operands[1]
• Support this here too
• Make the program more consistent by exiting directly via exit() instead of mixing calls to exit() and $self->exit() (definition of custom exit() can be eliminated)
• Behave more like BSD version by printing usage for bare "rm" with no -f and no args

1. https://pubs.opengroup.org/onlinepubs/009695399/utilities/rm.html

[briandfoy]

changes: -f with no arguments exits with 0 and shows usage