commit c87e46881 ("MINOR: http-rules: Add a flag on redirect rules to know the
rule direction") introduced a new flag for redirect rules, but its value has
bits in common with REDIRECT_FLAG_DROP_QS, which makes us enter this code path
in http_apply_redirect_rule(), which will then drop the query string.
To fix this, just give REDIRECT_FLAG_FROM_REQ its own unique value.
This must be backported where c87e468816 is backported.
This should fix issue 521.