NodeUtils: do not pass stdin to group upcalls

[martinetd]

Some commands consume stdin when they shouldn't, but we shouldn't have been giving stdin to upcalls in the first place.

Reported-by: Kilian Cavalotti [email protected]

replaces #580

[degremont]

Maybe we should add a line in the right place in documentation to clarify that?

Otherwise, LGTM!

[martinetd]

Hmm, documentation doesn't seem to say much about how upcalls are executed currently, I think it's fine as it is.

Group source upcalls in doc/sphinx/config.rst could perhaps say something (e.g. "upcalls' standard output is used if the command returns 0, standard input is closed and standard error is shared with the caller"), especially the part about requiring success execution now that I noticed it, I don't know.

[degremont]

Actually you're right, this is not that important.