Linux internal attach fails for non-CLONE_SIGHAND and non-CLONE_THREAD threads

Xref original #722.

We've known this as we rely on only iterating /proc/self/task but never filed an official issue.

Fortunately: "Since Linux 2.5.35, flags must also include CLONE_SIGHAND if CLONE_THREAD
is specified."

Thus we are only worried about non-CLONE_THREAD threads. If they have CLONE_SIGHAND we could presumably do a simple tweak to look in /proc/self/task/nnn/children and then recurse from there.

If they do not have either flag, we only have two reasonable solutions: craft some ptrace-based solution where we fork a helper process, or use app cooperation.