BUG/MEDIUM: wdt: Don't ignore WDTSIG and DEBUGSIG in __signal_process_queue().

BUG/MEDIUM: wdt: Don't ignore WDTSIG and DEBUGSIG in __signal_process_queue().

When running __signal_process_queue(), we ignore most signals. We can't,
however, ignore WDTSIG and DEBUGSIG, otherwise that thread may end up
waiting for another one that could hold a glibc lock, while the other thread
wait for this one to enter debug_handler().
So make sure WDTSIG and DEBUGSIG aren't ignored, if they are defined.
This probably explains the watchdog deadlock described in github issue

This should be backported to 2.1, 2.0 and 1.9.