Allow to query for server extensions available.

[dkocher]

Fix for #590.

[hierynomus]

Shouldn't we also be checking the server extensions when doing extended requests?

[dkocher]

Shouldn't we also be checking the server extensions when doing extended requests?

I think that should be the responsibility of the user. We use it like

if supportsServerExtension("statvfs", "openssh.com")
    newExtendedRequest("[email protected]")

[hierynomus]

Shouldn't we also be checking the server extensions when doing extended requests?

I think that should be the responsibility of the user. We use it like

if supportsServerExtension("statvfs", "openssh.com")
    newExtendedRequest("[email protected]")

It would actually be good to guard against sending bad requests to the server, as we know which extended requests are available. I'll add that in a separate PR and merge this one, as it is a pre-existing situation.