Fixes a regression from edb2eed6 where all signals, including SIGSEGV and SIGBUS, were blocked while running exit code. This meant that DR wouldn't catch and report crashes, and more importantly clients couldn't use safe_read and other features in their exit events. Now we avoid blocking those two key signals.
Tested: Dr. Memory's safe_read uses during exit no longer crash or hang.
Fixes #3839 (closed)