Add early command line parsing

[pjonsson]

This is a different approach for the infrastructure parts of #755.

This pull request allows each compilation unit to specify their own command line options, and switches the border-router to use the new API.

The compilation units add their options before main is run, and parsing command line options is the first thing that happen inside main, so command line options can be used to configure things before processes are started.

The code uses getopt_long_only so legacy parameters (-v0..-v5) still work. The getopt_long_only function is widely available, but if the SDK for some platform does not provide that function, users can put in the code from newlib/picolibc or one of the BSDs and things should work.

This PR is already quite a few lines so there is no conflict detection, I'm sure calling getopt_long with two options that have the same name will give "interesting" results.

[pjonsson]

@tdesmet do you think this PR and adding the following to some compilation unit in your software would work for your use case from #755?

CC_CONSTRUCTOR(CONTIKI_MIN_INIT_PRIO - 1) static void
init_key_config_data(void)
{
  // Do whatever initialization before main().
}

static int
key_callback(char *optarg)
{
  if(strlen(optarg) < 17) {
    fprintf(stderr, "Key must be at least 17 characters long\n");
    return 1;
  }
  key_value_thing = optarg;
  return 0;
}

CONTIKI_OPTION(CONTIKI_VERBOSE_PRIO + 6, {"key", required_argument, NULL, 0},
               key_callback, "key value\n");

[simonduq]

Hi!

This was my mistake, very sorry about it, re-opening this PR now.

What happened is the following: I was switching the base branch for this repository https://github.com/wittra/contiki-ng from develop to wittra. But accidentally, I made the change on the wrong repo (this repo) and instead of switching the base branch I renamed it. And somehow github deleted develop and closed all PRs...

Many apologies for this mishap 🙏; I haven't contributed in a while.. but now at least everybody got some notification from me :p

[pjonsson]

The ordering guarantees for the priority to CC_CONSTRUCTOR on macOS is weaker, only guaranteed within the module. I have updated the PR so it also works on macOS, even if more modules with command line parameters are added.