Moves signal handler installation from init time to start-execution time. This avoids a problem with split init-vs-start where we install our handler, then remove it to go back native between init and start, and then reinstate it. We need to re-install our handlers every start in any case, so by moving the init time installation to the same point, we simplify the code.
We keep the init-time SIGSEGV and SIGBUS crash handlers active longer, to cover crashes later in init, especially in client init.
Fixes #2335 (closed)