Convert lib/sshutils to use slog

[rosstimothy]

In addition to the conversion, the reverstunnel server now has a slog.Logger provided so that it can be passed into the sshutils.Server, is not used otherwise. A similar migration will be performed in the future for the reversetunnel package.

[espadolini]

Should some of those context.Background() be context.TODO() instead?

[espadolini]

Should we keep this in a group? TBH I've never paid attention to this in our use of logrus.

[rosstimothy]

A group might be fitting, though what would the appropriate key be? scp?

[rosstimothy]

Should some of those context.Background() be context.TODO() instead?

From RFD 154:

If no context is readily available for the logger to consume, prefer to use a context.TODO if it is possible to get a context to the call site in the future but refactoring is to do be done in a separate unit of work. If there is likely to never be a context provided to the function producing the log output, then prefer to use a context.Background to indicate as such.

I suppose it may be possible to get a context into some of the places, but for others it would likely require a fair bit of refactoring.