So there still seem to be some quirks with stepping through signal
trampolines. But before we decide what's the bug, I think we should
ask ourselves what the desired behaviour is. In my view:
* "finish" from within a signal handler should make us return to the
signal trampoline.
* "finish" from within a signal trampoline should make us return to
the point where the signal interruption occured.
* "stepi" from within a signal handler should step through the signal
handler and back into the signal trampoline.
* "stepi" from within a signal trampoline should step through the
signal trampoline until the sigreturn system call.