Encode semantics of Session::keepalive_send in return type

Issue

The current implementation of Session::keepalive_send returns Result<u32, Error>. Users might suspect that if they receive an Ok(seconds) they should act on it and send another keep alive message after seconds has elapsed.

However, this is not the case if keepalive is disabled for the session since Session::keepalive_send will return Ok(0) indicating that keepalive is not set. Here's where that happens in libssh2:

https://github.com/libssh2/libssh2/blob/a88a727c2a1840f979b34f12bcce3d55dcd7ea6e/src/keepalive.c#L62-L66

    if(!session->keepalive_interval) {
        if(seconds_to_next)
            *seconds_to_next = 0;
        return 0;
    }

Possible Solutions

The most lightweight thing we can do is update the documentation for Session::keepalive_send so that it clearly indicates that users will get an Ok(0) back if keep alive is not set.

Alternatively, if we're alright with a breaking change for the next minor version release we could change the return type to Result<Option<NonZeroU32>, Error>, or Result<Option<Duration>, Error> and set the option to None if the C api returns zero. I think this is the best way to prevent accidental misuse of the API, but it does require a breaking change.

Start at Session::keepalive_send in session.rs and compare its documented return value with the keepalive.c behavior linked in the issue. Confirm whether the documentation-only option is preferred, then make the result explicit for the disabled-keepalive case; done means users can tell that Ok(0) means no keepalive is configured.