MINOR: wdt: Move the definitions of WDTSIG and DEBUGSIG into types/signal.h.

MINOR: wdt: Move the definitions of WDTSIG and DEBUGSIG into types/signal.h.

Move the definition of WDTSIG and DEBUGSIG from wdt.c and debug.c into
types/signal.h, so that we can access them in another file.
We need those definition to avoid blocking those signals when running
__signal_process_queue().

This should be backported to 2.1, 2.0 and 1.9.