Simplify double pointer usage in the CLI

[CluEleSsUK]

There are a few uses of double pointers in the CLI to set a ChainInfo object, such as: https://github.com/drand/drand/blob/1ec6042e98439716ad1beb140958cb9e482323b4/cmd/client/lib/cli.go#L162 and https://github.com/drand/drand/blob/1ec6042e98439716ad1beb140958cb9e482323b4/cmd/client/lib/cli.go#L194.

Double pointers harm readability and ability to reason about control flow, so these should be simplified into another form.