When executing vcl.show for labels, show the VCL the label points to
instead of asserting.

segments dominate the _.index file.

Relevant for #3023

Be a bit more elaborate on the expected behaviour

Submitted by:	Frederic Lecaille

It's really not obvious, and if it weren't for other tests covering
probes it wouldn't be enough by itself to verify what it needs to.

And we have a goal of making the VSL self-documenting so reordering
anything is pointless. We already reorder SLTs in the documentation
where sorting really matters to end users.

With bonus code style OCD fixes. Better diff with the --ignore-all-space
option.

Not really spotted by:	Coverity