place errors on stderr, instead of stdout

[AladW]

Right now, aria2c prints diagnostics on stdout, which makes it more difficult to use in scripts (in particular, shell pipes). curl and wget print to stderr instead.

[tatsuhiro-t]

I'd like to make this WONTFIX. This is because if we change the device, the existing script might not work anymore.

[AladW]

As with #639, you could keep the existing behaviour by making the new one optional, e.g via --stderr.

[tatsuhiro-t]

Yes, we can send all stuff currently written to stdout to stderr with new --stderr option.
But it only happens after --stderr is parsed correctly. Before options are parsed, still stdout is used for backward compatibility.

[tatsuhiro-t]

Fix committed via db239c2

[AladW]

Works for me, thanks. Though it seems the option is not included in aria2c --help.

edit: works with aria2c '-h#advanced'