DEBUG: implement 4 levels of choices between warn and crash.

We used to have DEBUG_STRICT_NOCRASH to disable crashes on BUG_ON().
Now we have other levels (WARN_ON(), CHECK_IF()) so we need something
finer-grained.

This patch introduces DEBUG_STRICT_ACTION which takes an integer value.
0 disables crashes and is the equivalent of DEBUG_STRICT_NOCRASH. 1 is
the default and only enables crashes on BUG_ON(). 2 also enables crashes
on WARN_ON(), and 3 also enables warnings on CHECK_IF(), and is suited
to developers and CI.
diff --git a/Makefile b/Makefile
index 28de8a3..964b7fb 100644
--- a/Makefile
+++ b/Makefile
@@ -255,7 +255,7 @@
 # not use them at all. Some even more obscure ones might also be available
 # without appearing here. Currently defined DEBUG macros include DEBUG_FULL,
 # DEBUG_MEM_STATS, DEBUG_DONT_SHARE_POOLS, DEBUG_FD, DEBUG_POOL_INTEGRITY,
-# DEBUG_NO_POOLS, DEBUG_FAIL_ALLOC, DEBUG_STRICT_NOCRASH, DEBUG_HPACK,
+# DEBUG_NO_POOLS, DEBUG_FAIL_ALLOC, DEBUG_STRICT_ACTION=[0-3], DEBUG_HPACK,
 # DEBUG_AUTH, DEBUG_SPOE, DEBUG_UAF, DEBUG_THREAD, DEBUG_STRICT, DEBUG_DEV,
 # DEBUG_TASK, DEBUG_MEMORY_POOLS, DEBUG_POOL_TRACING.
 DEBUG = -DDEBUG_STRICT -DDEBUG_MEMORY_POOLS