AddPersistentFlag adds a persistent flag `flag“, with default `def“ and usage message `usage`, and it will also bind the flag to the viper `configKey`.

CheckConnectAddr checks if it's possible to connect to a TCP address, and will return an error if anything is wrong.

CheckFilesExist checks if the given files exist, and are not directories.

CheckListenPort checks if it's possible to listen on an address/port combination, and will return an error if anything is wrong.