MINOR: ist: add ist_find_http_ctl() to match http CTL chars

MINOR: ist: add ist_find_http_ctl() to match http CTL chars

Some tests in the HTTP code involve ist_find_ctl() hoping to find any
HTTP CTL char, but CTL for ist is 0x00..0x1F only (and documented as
such so we won't change it). For HTTP, this also includes 0x7F which
these tests are missing.

It's not realistic to do a second full scan on those data for something
that never happens. Better introduce a new function which scans for
0x00..0x1F,0x7F, that's much more efficient. It's constructed exactly
like the existing one with a extra check for 0x7F by adding 1 to
generate a carry in this case as well. It's expected to cost roughly
2 extra cycles per word, which is OK.