The alloc exec and filesystem/logs commands allow passing the `-job` flag to
select a random allocation. If the namespace for the command is set to `*`, the
RPC handler doesn't handle this correctly as it's expecting to query for a
specific job. Most commands handle this ambiguity by first verifying that only a
single object of the type in question exists (ex. a single node or job).
Update these commands so that when the `-job` flag is set we first verify
there's a single job that matches. This also allows us to extend the
functionality to allow for the `-job` flag to support prefix matching.
Fixes: #12097
