Add server support for signal channel (RFC 4254, 6.9)

[Jay Weisskopf]

I would like to see sshd support the signal channel. Issue 1424 [1] has
been tracking this for 10 years, but it's not very clear to me why it never
progressed. Discussion in that issue centers around the ergonomics of
adding support to the client. I am more interested in seeing this work with
other clients that already implement the feature (e.g. Go's SSH package
[2]).

I attached a smaller patch that only makes changes to the server. It is a
subset of the existing attached patch.

Thanks,
Jay

[1]: https://bugzilla.mindrot.org/show_bug.cgi?id=1424#c44
[2]: https://github.com/golang/go/issues/16597