MEDIUM: http: add the "short-status-code" violation exception

MEDIUM: http: add the "short-status-code" violation exception

Now by default we reject status codes shorter than 3 digits, though
this exception permits to continue to accept them. There is no valid
reason for accepting these and no known client currently supports
them, but it has been supported due to historical reasons that are
long outdated.

It is expected that noone will notice, and may be candidate for
backporting only to the latest LTS version after some observation
time.