Fixes issues from 240b75d6 #2592 (closed) which added UNIX stack overflow reporting:
- Fixes -stack_size and -signal_stack_size to have a minimum of just the page size, not double it, as the guards are added on top.
- Fixes leak reporting to consider -stack_guard_pages and not just -guard_pages.
- Makes the client.stack-overflow test less flaky by setting a specific stack size to avoid variations in compilers and platforms.
Fixes #2606 (closed)