[Toybox] sntp -m design/documentation (optional arguments help syntax?)

[Rob Landley]

RFC 4330 (and newer) say that the default multicast address for broadcast sntp
time updates is 224.0.1.1 so I want "sntp -m" to have an optional argument
overriding that, but otherwise take the default.

Unfortunately, "optional aguments" exist but aren't used much. For example
--color is technically --color=auto (and can also take --color=always and
--color=never)... but actualy looking at what's after the = is still a todo item
for ls. (I think I might have done it in grep? I'd have to check. I _do_ have a
note that I should try to factor out common code there into lib...)

For optional longopts, you have to go =argument, for optional short options you
have to collate, in this case it would look like:

  sntp -m224.0.1.1

If you have a space it instead thinks you're going "sntp 224.0.1.1 -m", because
that's how optional arguments work. (And yes this means you can sntp -sM but not
sntp -Ms .)

The _problem_ is, even though I have this mechanism... I have no idea how to
usefully describe in the help text so users know what to do? I don't really have
a "usage: blah [-abc] ARG..." syntax for it, and even if I did it's not in
widespread enough use I'd expect people to pick it up? I added a "[-mADDRESS]"
entry to the command line but it's too subtle...

Not sure what to do about that. For the moment, I guess -m requires the
argument? (Yes, somebody is still using multicast for something. In 2019. I
know, but there's a user waiting...)

Trying to cut a release and promoting sntp is the most prominent remaining todo
item...

Rob