statd: add support for additional operstates

[rical]

Add support for yang link state "lower-layer-down" and "not-present".

[troglobit]

Overall better solution to any of what we have right now. I did a few more fixes in #82 that I'd like to get in, but I could also rework that PR.

Just a thought, instead of multiple strcmp(), personally I'd go for more of a table of strings:

struct {
  char *kern;
  char *yang;
} map[] = {
  { "foo", "bar" },
};

for (size_t i = 0; sizeof(map) / sizeof(map[0]); i++) {
  if (strcmp(operstate, map[i]))
    continue;
  return map[i];
}

return "unknown";

[troglobit]

Nice work! 👍

[troglobit]

Btw, I'll be back from vacation in a couple of week, but iirc @wkz will be back already next week, Monday.