DolphinTool: Show choices as metavars, and don't lie about defaults

The previous `%choices` string no longer works since cpp-optparse got updated. In a confusing turn of events, I believe cpp-optparse got renamed to cpp-argparse at some point in history, and a new cpp-optparse repository was made in its place as a fork of argparse before `%choices` was added to it, and picked up development without ever adding `%choices`. This lead to the breaking change not being documented anywhere in the commit history.

Showing it as a metavar matches the style of many GNU tools, this is how it renders:

```
$ ./Binaries/dolphin-tool verify -h
Usage: verify [options]...

Options:
  -h, --help            show this help message and exit
  -u USER, --user=USER  User folder path, required for temporary processing
                        files. Will be automatically created if this option is
                        not set.
  -i FILE, --input=FILE
                        Path to input file.
  -a crc32|md5|sha1|rchash, --algorithm=crc32|md5|sha1|rchash
                        Optional. Compute and print the digest using the
                        selected algorithm, then exit.
```

Also we were lying about RVZ being a default value in the convert command.

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