Fix a crash when running as systemd service

- Adapted QuickExit/exit(0) calls to use EXIT_SUCCESS, which is more portable
- Fix a NULL dereference if a signal is raised in the constructor of the
class InspIRCd
- Resolve the parent's process exit code being SIGTERM=15 and typically not
EXIT_SUCCESS thus

A combination of the latter two points caused crashes when running as systemd unit.