BUG/MEDIUM: http: continue to emit 503 on keep-alive to different server

BUG/MEDIUM: http: continue to emit 503 on keep-alive to different server

Finn Arne Gangstad reported that commit 6b726adb35 ("MEDIUM: http: do
not report connection errors for second and further requests") breaks
support for serving static files by abusing the errorfile 503 statement.

Indeed, a second request over a connection sent to any server or backend
returning 503 would silently be dropped.

The proper solution consists in adding a flag on the session indicating
that the server connection was reused, and to only avoid the error code
in this case.