fix: "device update --locked" should also unlock

[displague]

Fixes #484

Previous behavior was to set {"locked":true} in PUT requests whenever --locked was specified. This is because the presence of the flag itself is treated as a toggle. The value argument was ignored unless given as --locked=true or --locked=false. When --locked=false was given, the PUT request was {}, since the code required true values to add the field to the update request.

This change technically breaks the current ability to do metal device update --locked --some-other-arg.

$ PACKNGO_DEBUG=1 go run ./cmd/metal device update --locked false --locked true --id 1234
Error: parameter locked may only be set once

$ PACKNGO_DEBUG=1 go run ./cmd/metal device update --locked --id 1234                    
Error: invalid argument "--id" for "-l, --locked" flag: strconv.ParseBool: parsing "--id": invalid syntax
...

$ PACKNGO_DEBUG=1 go run ./cmd/metal device update --locked true  --id 1234              
PUT /metal/v1//devices/1234 HTTP/1.1
...
{"locked":true}

$ PACKNGO_DEBUG=1 go run ./cmd/metal device update --locked false  --id 1234
PUT /metal/v1//devices/1234 HTTP/1.1
...
{"locked":false}