Alloc exec only works when task is passed as a flag and not an arg.
Alloc logs currently accepts either, but alloc signal and restart only
accept task as an arg. This adds -task as a flag to the other alloc
commands to make the cli UX consistent. If task is passed as a flag and
an arg, it ignores the arg.

This command will be used to send a signal to either a single task within an
allocation, or all of the tasks if <task-name> is omitted. If the sent signal
terminates the allocation, it will be treated as if the allocation has crashed,
rather than as if it was operator-terminated.

Signal validation is currently handled by the driver itself and nomad
does not attempt to restrict or validate them.

This adds a `nomad alloc stop` command that can be used to stop and
force migrate an allocation to a different node.

This is built on top of the AllocUpdateDesiredTransitionRequest and
explicitly limits the scope of access to that transition to expose it
under the alloc-lifecycle ACL.

The API returns the follow up eval that can be used as part of
monitoring in the CLI or parsed and used in an external tool.