Some methods currently support a single platform only, but we may
be able to support multiple platforms.
This patch prepares the option-structs for multi-platform support,
but (for now) returning an error if multiple options are provided.
We need a similar check on the daemon-side, but still need to check
on the client, as older daemons will ignore multiple platforms, which
may be unexpected.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>