click/string: make mutable_data more explicit about memo refcount

Previously, String::mutable_data might deref but then continue to use
_r.data and _r.length.  It would prevent the underlying memo_t from
disappearing by taking a temporary reference via a local String.

This change makes that intention more explict by using the temporary instead
of accessing the now invalid _r directly.
click/linux: add dump stack to click_assert_failed

Now when an assertion fails, we will dump stack even if we don't panic.
This is crazy useful.