DEBUG: extend DEBUG_STRESS to ease testing and turn on extra checks

DEBUG: extend DEBUG_STRESS to ease testing and turn on extra checks

DEBUG_STRESS is currently used only to expose "stress-level". With this
patch, we go a bit further, by automatically forcing DEBUG_STRICT and
DEBUG_STRICT_ACTION to their highest values in order to enable all
BUG_ON levels, and make all of them result in a crash. In addition,
care is taken to always only have 0 or 1 in the macro, so that it can be
tested using "#if DEBUG_STRESS > 0" as well as "if (DEBUG_STRESS) { }"
everywhere.

The goal will be to ease insertion of extra tests for builds dedicated
to stress-testing that enable possibly expensive extra checks on certain
code paths that cannot reasonably be compiled in for production code
right now.