Import security/ruby-net-ssh-multi into pkgsrc.

Net::SSH::Multi is a library for controlling multiple Net::SSH
connections via a single interface. It exposes an API similar to that
of Net::SSH::Connection::Session and Net::SSH::Connection::Channel,
making it simpler to adapt programs designed for single connections to
be used with multiple connections.

This library is particularly useful for automating repetitive tasks
that must be performed on multiple machines. It executes the commands
in parallel, and allows commands to be executed on subsets of servers
(defined by groups).

(jperkin)