client: New option -r for restricted mode

client: New option -r for restricted mode

Redirections and the execute command let the user read and write files
and run programs on the local system.

Restricted mode prevents such access.  This is useful when you want to
grant somebody access to just Empire, but not to the host system's
user account that runs the client.

Signed-off-by: Marisa Giancarla <fstltna@me.com>
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>