dnctl: Support reading config from file like ipfw(8)

[kp]

Summary

Extend the dnctl (dummynet config) tool to be able to read commands from
a file, just like ipfw already does.

Sponsored by: Rubicon Communications, LLC ("Netgate")

[cy]

Looks like there's some whitespace adjustment included too. Will that be a separate commit?

[kp]

In D33627#760102, @cy wrote:

Looks like there's some whitespace adjustment included too. Will that be a separate commit?

I didn't intend for there to be any, other than increasing the indent level of the while ((c = getopt(ac, av, "cfNnp:qS")) != -1) loop, and as far as I can tell that's the only whitespace change.