CONTRIB: debug: add the possibility to decode the value as certain types only

CONTRIB: debug: add the possibility to decode the value as certain types only

It's often confusing to have a whole dump on the screen while only
checking for a set of task or stream flags, and appending "|grep ^chn"
isn't very convenient to repeat the opeation. Instead let's add the
ability to filter the output as certain types only by prepending their
name(s) before the value.

(cherry picked from commit 354b6f5e288f466aed48d46dd8467b023a0b3660)
Signed-off-by: Willy Tarreau <w@1wt.eu>