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