Migrate examples from docopt to structopt.

[ratmice]

Looking at the examples, I noticed that according to the docopt README, "This crate is unlikely to see significant future evolution.", as well as "Consider using clap or possibly structopt instead."

It didn't seem like it'd be difficult to do, so I figured i might as well go ahead and cold-call it.
However this has only undegone light testing.

Notes:

• log.rs: had to reorder struct members to get the multiple varargs working.
• various: --foo --bar I'd tried using a name="n" long="foo", this worked, unless there were 2 things wanting to be named n. So I partially backed that, but only in the places where e.g. occurred multiple times.
• all: rather than rename struct members from flag_... and arg_... naming convention, I used the name = "...", this lead to a much cleaner patch. I could do a search/replace over s/flag_// and remove the corresponding name="..." in a follow up if this patch is wanted and that is preferred.