In the Linux kernel, the BUG() macro causes an instant panic. Our
BUG() macro is different, however: it generates a nonfatal assertion
failure, and is usable as an expression.
Additionally, this patch tells util_bug.h to make all assertion
failures into fatal conditions when we're building with a static
analysis tool, so that the analysis tool can look for instances
where they're reachable.
Fixes bug 23030.
Unlike tor_assert(), these macros don't abort the process. They're
good for checking conditions we want to warn about, but which don't
warrant a full crash.
This commit also changes the default implementation for
tor_fragile_assert() to tor_assert_nonfatal_unreached_once().
Closes ticket 18613.
