FlagToEnv converts flag string to upper-case environment variable key string.

IsPSet checks if flagset has set but with pflag.

IsSet checks if flagset has set.

SetFlagsFromEnv parses all registered flags in the given flagset, and if they are not already set it attempts to set their values from environment variables.

SetPflagsFromEnv is similar to SetFlagsFromEnv.

DeprecatedFlag encapsulates a flag that may have been previously valid but is now deprecated.

IgnoredFlag encapsulates a flag that may have been previously valid but is now ignored.