evaluate: allow to re-use existing metered set

evaluate: allow to re-use existing metered set

Blamed commit translates old meter syntax (which used to allocate an
anonymous set) to dynamic sets.

A side effect of this is that re-adding a meter rule after chain was
flushed results in an error, unlike anonymous sets named sets are not
impacted by the flush.

Refine this: if a set of the same name exists and is compatible, then
re-use it instead of returning an error.

Also pick up the reproducer kindly provided by the reporter and place it
in the shell test directory.

Fixes: b8f8ddfff733 ("evaluate: translate meter into dynamic set")
Reported-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>