Fix: repeated args should not append (but overwrite or ignore) if already have enough params

[xparq]

With moons taking 1 param, args-test.exe --moons=11 --unknown=123 --moons=-99 is now incorrect:

NAMED:
moons = 11, -99
unknown = 123

• The new moons should be ignored if it's a multi-value arg. and already has enough:

  --moons=1,2,3 --unknown=123 --moons=4,5 --moons=6

Should be 1, 2, 3 + error flags.
-> But... Now (after a ~year), however, I think the "it doesn't yet have enough..." case shouldn't actually be supported! -> #44 instead!

• And it should overwrite if it's a single-val option.
  • Probably also, in general, if it takes a fixed number of params: --moons=1,2,3 --junk --moons=4,5,6?
• But what to do if they are greedy: --moons=1,2 --junk --moons=3 -x --moons=4,5,6? Shouldn't this accumulate all the params then? I feel that greediness is a weaker factor, though, than consistency (with the other arities).

---

Actually...

• There should just be a rule for this (similarly to arity), with Override as default, Append as an option...

[xparq]

Got closed prematurely: overriding --arg (by repeating it) only works if
a) no positional args follow --arg (to be repeated), or
b) --arg is defined to have fixed number of params!