GitLab

In case of multiple outputs, the minimum successful write length is
returned.  In line with this, in case any output has a write error, zero is
returned.

In case of no outputs, -1 is returned.

The return value can be used to assess whether writes were attempted, and
if so, whether they succeeded.
Signed-off-by: Maarten van der Schrieck <maarten@thingsconnected.nl>