Use restrict key option with svn-{packages,community}

No functional change; the "restrict" key option is a shorthand for:

- no-agent-forwarding
- no-port-forwarding
- no-X11-forwarding
- no-pty
- no-user-rc

It was added in OpenSSH 7.2 (2016-02-29) as a convenient way to specify
an authorized key should have "all current and future key restrictions"
applied to it.