option to not override signal handler

See also https://github.com/jupyter/qtconsole/issues/490

Hey there,

how can I embed qtconsole into an application without it overriding my own application's custom sigint handler?

Assume you have code that does

```
std::signal(SIGINT, &handleSigInt);
...
// do not initialize signal handlers
const auto initsigs = 0;
Py_InitializeEx(initsigs);
```

Then it is embedding qtconsole eventually, but at that point the sigint handler will be unset:

```
Thread 1 "QiTissue" hit Breakpoint 3, __interceptor_sigaction (signum=2, act=0x7fffffff8a60, oldact=0x7fffffff8b00)
at /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc:55
55 in /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc
(gdb) bt
#0 __interceptor_sigaction(int, __sanitizer::__sanitizer_sigaction const*, __sanitizer::__sanitizer_sigaction*)
(signum=2, act=0x7fffffff8a60, oldact=0x7fffffff8b00)
at /build/gcc/src/gcc/libsanitizer/sanitizer_common/sanitizer_signal_interceptors.inc:55
#1 0x00007fff5c055f0f in PyOS_setsig (sig=sig@entry=2, handler=handler@entry=0x7fff5c0b6fec )
at Python/pylifecycle.c:2536
#2 0x00007fff5c0b849f in signal_signal_impl
(module=, handler=, signalnum=2) at ./Modules/signalmodule.c:505
#3 signal_signal (module=, args=, nargs=)
at ./Modules/clinic/signalmodule.c.h:141
#4 0x00007fff5bf7bbf8 in cfunction_vectorcall_FASTCALL
(func=, args=0x7fff58a24a88, nargsf=, kwnames=) at Objects/methodobject.c:426
```

or in python terms:

```
(gdb) py-bt
Traceback (most recent call first):

File "/usr/lib/python3.9/signal.py", line 47, in signal
handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
File "/home/milian/.local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 280, in pre_handler_hook
self.saved_sigint_handler = signal(SIGINT, default_int_handler)
File "/home/milian/.local/lib/python3.9/site-packages/ipykernel/kernelbase.py", line 257, in dispatch_shell
self.pre_handler_hook()
```

the kernelbase.py code seems to reset the signal handler unconditionally to some default which is highly unexpected to me:

```
def pre_handler_hook(self):
"""Hook to execute before calling message handler"""
# ensure default_int_handler during handler call
self.saved_sigint_handler = signal(SIGINT, default_int_handler)
```

Is there a way to prevent this from happening?