MINOR: http: use ist_find_http_ctl() instead of ist_find_ctl()

MINOR: http: use ist_find_http_ctl() instead of ist_find_ctl()

At a few places we were using ist_find_ctl() as an accelerator to locate
possibly forbidden CTL chars, but that one doesn't detect the DEL char,
so now that we have the new ist_find_http_ctl(), let's use it instead.
This is not tagged as a bug because nothing at this point is made with
this char so this doesn't make a difference at this point.