If we know that the remote host is a unix system, and that it uses some
version of the posix shell, then we can adjust our command line to cd to
the requested directory (as set by OSC 7) and then exec the requested
command.
That's what SshDomain::assume_unix indicates and what this commit does.
This puts us in a better position for the future to be able
to configure whether we use wezterm, tmux or no multiplexing.
Today we allow wezterm or no multiplexing.
Add docs on this new setting.
refs: https://github.com/wez/wezterm/issues/1456