Various minor polish to VSM area.

Still not done, in particular: Do not roll any releases until
libvarnishapi symbo/version stuff has been polished.

Fixes	#829

Instead of the diag() callback, use a vsb to collect diagnostics
and return it with VSM_Error().

Simpler logic around open/close.

More useful Abandonned() and StillValid() functions.

Still not happy with __itern(), will revisit later.

1060 was fixed with bdbb1d59

Fixes	#971

At least "work" in the sense that varnishtest does not obviously
break, but since it doesn't test the utils at all (but does use
the varnishapi) more work & testing will be needed.

and copy them to the "real vsm" when we create it.

it via heritage.

again.  Much still outstanding.

but varnishapi and users do not work yet.

other way than -l does you no good.

of the fact that we have so many params in units of bytes which
you cannot tell "64k" so now you can.

Notice that a couple of specialist params (fetch_chunksize and
fetch_maxchunksize) have changed units from kilobytes to bytes,
but the default values are the same.

Fixes	#1060