cmd/utils: fix handling of boolean flags when they are set to false

[cuiweixie]

geth --nodiscover=false
may result in ctx.IsSet(NoDiscoverFlag.Name) is true, but cfg. NoDiscovery should be false, not true.

[gballet]

Hi, thanks a lot for your contribution! 🙏
At the moment, the go-ethereum team is overloaded with pull requests and in order to ensure we can give high quality review and attention to critical changes, we are not accepting refactoring-only PRs or minor upgrades.

We really appreciate the effort and encourage you to keep contributing, but for now we need to focus our limited review bandwidth on feature work, bug fixes, and protocol changes.

Thanks for understanding!

[jwasinger]

This is worth merging.