fix the GRASS v_net modules

[gioman]

Description

For some reason in QGIS 3 a fundamental "-s" flag was not added in the v.net GRASS modules.

Checklist

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[nyalldawson]

Can we get some unit tests here?

[alexbruy]

@gioman unfortunatelly this does not fix all issues, e.g. v.net still failed to produce output even with this change. I was working on more complete PR (#9046) and did not noticed that you already submitted this.

[gioman]

@alexbruy redmine is slow sending out notifications, so I didn't noticed https://issues.qgis.org/issues/19904#note-13. Anyway as my patch is not complete we can close.