CLI import help changes (rebased onto develop)


This is the same as gh-2766 but rebased onto develop.

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Another set of changes for the CLI import suggested by @manics.
To test this PR, check the ouput of

- ``bin/omero --java-help`
- ``bin/omero --advanced-help``

                
`bin/omero import` needs `--`; however, `./importer-cli` does *not*. (good catch)