jlink --add-options plugin does not allow GNU style options to be provided

The jlink options parsing currently doesn't allow the argument for a jlink plugin to use GNU style (--) options, e.g.

$ jlink --add-modules java.base,jdk.incubator.concurrent --output myimage --add-options --add-modules=jdk.incubator.concurrent
Error: no value given for --add-options
Usage: jlink <options> --module-path <modulepath> --add-modules <module>[,<module>...]
Use --help for a list of possible options

A possible workaround is to prepend a space, e.g.
--add-options \ --add-modules=jdk.incubator.concurrent

The parsing needs to be ex-examined. It may be that --add-options should have @argfile support, or maybe it should support quoting so the arguments for a jlink option can be delimited by quotes.