Use structopt crate for command line programs

[realcr]

We currently use the clap crate for command line parsing. clap uses strings to refer to arguments, which is not checked at compile time. The structopt builds over clap and allows to define structs which automatically turn into command line arguments.

This should affect code in:

• offst-bin (All binaries)
• offst-stctrl (The stctrl binary).

[realcr]

Done.