flag: should not output errors directly to stderr

[jvehent]

When using a flagset, errors are printed to stderr in addition to being returned by flag.Parse(). It is redundant, since the caller can print the error if desired. It also creates a small issue in a situation where multiple flagsets are used to parse a single command line.

I have a command line interface that provides a wrapper to multiple modules. The command line has its own flagset [1], and forward the flags it cannot parse to one of many packages where specific flagsets are defined [2, 3]. The issue is that the first call to Parse() detects the flags it doesn't know and prints an error to stderr directly. I would like to avoid printing this error. I ended up setting the output to os.DevNull around the call to Parse() to prevent displaying it to the user, but it feels like a hack [4].

The culpric is here: https://github.com/golang/go/blob/master/src/flag/flag.go#L793
I suggest removing that line and only returning the error to the caller through the Error value.

I'm not aware of a different way to split a flagset across packages, but if there's one, I'm very interested :)

[1] https://github.com/mozilla/mig/blob/master/src/mig/client/cmd/main.go#L71-L77
[2] https://github.com/mozilla/mig/blob/master/src/mig/modules/file/paramscreator.go#L285-L305
[3] https://github.com/mozilla/mig/blob/master/src/mig/modules/netstat/paramscreator.go#L171-L178
[4] https://github.com/mozilla/mig/blob/master/src/mig/client/cmd/main.go#L124

[minux]

Just use SetOutput on the flag set to set the output io.Writer to
ioutil.Discard.

Of course we can't remove that line, otherwise it will also change the
behavior of toplevel flag.CommandLine flagset.