respect argument separations when collecting separate values

[metsw24-max]

Separate values collected even when they are disallowed

Dropping the else in e319b42 left the value-collection loop in ParseArgsValues running unconditionally, so SetArgumentSeparations no longer reaches it: with separate values off, --jobs 3 on an ImplicitValueFlag takes 3 instead of leaving it positional, and --nums=1 2 fills a Nargs{2, 3} flag. The awkward one is with joined short values off, where -nf 5 now takes 5 as n's value, so ParseShort breaks out of the chunk and f is silently dropped with nothing reported. Gated the loop on separate values being allowed and on the joined value having actually been taken rather than discarded; the added test covers all three plus the joined-then-separate nargs case from e319b42, and fails on master.

[Taywee]

Good catch. Nargs is perpetually a weird corner case.

Thanks for the fix.