pmdalinux: ensure long help text entries wrap at 80 columns

Fixes qa/469 failure, after recent network metric text improvements.
pmdalinux: fix compiler warning in network struct zeroing
rc_pmcd: execute _pmda_setup in the background

Amer reports that use of the .NeedInstall mechanism for
PMDAs introduces longer image startup times due to the
rc_pmcd script performing PMDA installation serially.
There's no reason for that - this processing can be done
in the background as soon as pmcd has started (just like
we do with pmloggers in the rc_pmlogger script already).

Test qa/300 is tweaked to give a little more time before
verifying the .NeedInstall processing has / has not been
done.
pmda unbound: a new PMDA for the Unbound DNS resolver

This adds a PMDA for the Unbound DNS resolver. Currently exposing 140
metrics gathered from Unbounds "unbound-control stats" command.  It's
missing the per thread metrics, but I plan on adding these at a later
point.

Signed-off-by: Jan-Frode Myklebust <janfrode@tanso.net>
qa: correct some header comments for a couple of tests
pmda unbound: add initial tests for Unbound DNS resolver PMDA

This adds two tests for the new Unbound DNS resolver PMDA - one to
exercise the metric values being extracted from the unbound-control
command output, the other to exercise the Install/Remove process.
Found and fixed a bug in the latter, where an exception would be
raised if the executed command did not exist.

Also added a makefile for the unbound PMDA and generally stitched
it into the build/packaging, ready for the next release.
pmda dmcache: more elegant handling of missing dmsetup binary
qa: add unbound pmda qa data files into the build