MEDIUM: http: factorize the "auth" action of http-request and stats

MEDIUM: http: factorize the "auth" action of http-request and stats

Both use exactly the same mechanism, except for the choice of the
default realm to be emitted when none is selected. It can be achieved
by simply comparing the ruleset with the stats' for now. This achieves
a significant code reduction and further, removes the dependence on
the pointer to the final rule in the caller.