Use `sysvmq::id_from_key(key)` to receive the id of a queue identified
with `key`. Here an error check is added as well. Adapt
`Sysv::Unlink::run()` to comply with that change.
This adds the `sysvmq` package which handles SysV IPC message queues.
For consistency the parameter for `permissions` is renamed to `mode`
also for POSIX message queues.
