MINOR: tinfo: split the signal handler report flags into 3

MINOR: tinfo: split the signal handler report flags into 3

While signals are not recursive, one signal (e.g. wdt) may interrupt
another one (e.g. debug). The problem this causes is that when leaving
the inner handler, it removes the outer's flag, hence the protection
that comes with it. Let's just have 3 distinct flags for regular signals,
debug signal and watchdog signal. We add a 4th definition which is an
aggregate of the 3 to ease testing.

(cherry picked from commit fb7874c286dbe1594837fc75b9e96783dfe160f5)
Signed-off-by: Willy Tarreau <w@1wt.eu>