BUG/MINOR: http_ana: make sure redirect flags don't have overlapping bits

BUG/MINOR: http_ana: make sure redirect flags don't have overlapping bits

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 c87e46881687b8ddb9b3f459e60edb1e8d7c5d7c is backported.

This should fix issue 521.