MINOR: http: move redirect rule processing to its own function

We now have http_apply_redirect_rule() which does all the redirect-specific
job instead of having this inside http_process_req_common().

Also one of the benefit gained from uniformizing this code is that both
keep-alive and close response do emit the PR-- flags. The fix for the
flags could probably be backported to 1.4 though it's very minor.

The previous function http_perform_redirect() was becoming confusing
so it was renamed http_perform_server_redirect() since it only applies
to server-based redirection.
diff --git a/src/session.c b/src/session.c
index 3dc30d5..02632bf 100644
--- a/src/session.c
+++ b/src/session.c
@@ -2198,7 +2198,7 @@
 
 			srv = objt_server(s->target);
 			if (s->si[1].state == SI_ST_ASS && srv && srv->rdr_len && (s->flags & SN_REDIRECTABLE))
-				perform_http_redirect(s, &s->si[1]);
+				http_perform_server_redirect(s, &s->si[1]);
 		} while (s->si[1].state == SI_ST_ASS);
 
 		/* Now we can add the server name to a header (if requested) */