WIP(ptrace): Only use non-signal stack when using a default handler

This is a curious problem and it's really hard to solve it in a way
that doesn't feel hacky. On one hand, of course you want to be able to
modify and intercept what happens when you use a signal, right? On the
other hand, changes made to the context (especially singlestepping)
while a signal is handled (such as `SIGSTOP`) are not preserved since
the stack is restored after the signal handler was invoked.

I think what we have in this change makes sense anyway, as we don't
really want users modifying registers and other data in the default
signal behavior that occurs **in kernel mode**. Also trying to use
`PTRACE_SINGLESTEP` will set the singlestep flag only if in a
user-mode signal handler, else it will set it on the instruction after
the signal handling, which I guess makes sense since it can't affect
the kernel-mode code that runs the default handler.

I don't know. Help. Pls.