C-g in child frames

[Gerd Möllmann]

Hi Martin, just to let you know that I'm working on it, although
progress is slow.

Part of the problem seems to be that tty-top-frame in the child frame,
just after it has been opened with M-l, returns the root frame F1, not
the child F2. C-g then takes the top frame F1 as
internal_last_event_frame in handle_interrupt_signal, and so on.

F2 is actually the top frame for a short period of time during its
creation, but it gets reset to F1 by a switch-frame event, and I have no
idea yet where that comes from.