Add support for the special ${RANDOM} environment variable.

[erjoalgo]

This can be used to generate strings unique to each ssh connection. For example, it can be used to generate unique RemoteForward unix socket paths to avoid conflicts between multiple connections to a given remote host.

[erjoalgo]

For more details of this feature request: https://bugzilla.mindrot.org/show_bug.cgi?id=3609