Make repeatable args that can't take comma a special case

[lars-t-hansen]

This is obscure, but: not being able to write -j 123,345 for two jobs is just plain bad ux. But the comma separation is not allowed because it does not work with host name patterns: -h ml[1,3],ml9 can't be split at the comma without heroic effort. We should probably separate these cases: Commas should be disallowed for hostnames but allowed in all other places where it makes sense. This is still a footgun but it'll be less surprising overall. Long-term maybe we'll invest the heroic effort to fix it even for host names.

[lars-t-hansen]

To make this worse than I realized, -j 123,456 is OK currently (and is handled properly) because the arguments are integers and can't contain a comma.