Disambiguated the protocol used by the unixsock (and other) plugins.

Disambiguated the protocol used by the unixsock (and other) plugins.

As discussed with Florian on IRC, the interpretation of the status line
returned by the unixsock plugin (et.al.) is now strictly specified. A status
greater than or equal to zero indicates success and the number of subsequent
lines of output. A negative status still indicates failure.

This way, frontends can manage the output without having to know any
command-specific interpretation rules.

The GETVAL command has been updated to implement this specification. It now
returns each value on a separate line. collectd-unixsock(5) has been updated
to reflect the changes.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>