Rename VSM_FOREACH_SAFE as it isn't really safe in the vqueue sense,
and removing chunks from the VSM from the client library isn't
applicable anyway.

Make the VSM chunk marker sizes defines.

Check for shm open in VSM__itern

Revisit logic of VSM__itern: Make sure that the VSM_fantom is either
changed to the next, or not changed at all.

Add comments.

Keep copies of class, type and ident in VSM_fantom, and use these when
revalidating fantoms.

Do a proper VSM_Open in varnishstat

Add a VSM_ResetError function

Differentiate between missing HEAD marker and zero alloc sequence. The
latter is an indication Varnish isn't running, the former a corrupt
file.

Fix newline on diag messages for consistency.

Fix return value check inversion bug on stat call in VSM_Abandoned()

Don't assert on VSM_Open on an already open VSM. Report success instead.