Remove executable bit from sdb.in.
Reference Mono.Posix.dll.
Use Mono.Unix.UnixSignal for handling SIGINT.

As described in BXC #16444 [0], we cannot use managed signal
handlers because managed/native transitions perform all kinds
of signal-unsafe work. So use the Mono.Posix abstraction that
allows us to wait for a signal and then act on it in normal
managed code.

[0] https://bugzilla.xamarin.com/show_bug.cgi?id=16444