This adds an initial implementation of ACLs for HostVolumes.

Because HostVolumes are a cluster-wide resource, they cannot be tied to
a namespace, thus here we allow similar wildcard definitions based on
their names, tied to a set of capabilities.

Initially, the only available capabilities are deny, or mount. These
may be extended in the future to allow read-fs, mount-readonly and
similar capabilities.

This adds `alloc-exec` capability to allow operator to execute command into a
running task.  Furthermore, it adds `alloc-node-exec` capability, required when
the alloc task is raw_exec or a driver with no FSIsolation.

This capability will gate access to features that allow interacting with
a running allocation, for example, signalling, stopping, and rescheduling
specific allocations.

This commit adds basic support for globbing namespaces in acl
definitions.

For concrete definitions, we merge all of the defined policies at load time, and
perform a simple lookup later on. If an exact match of a concrete
definition is found, we do not attempt to resolve globs.

For glob definitions, we merge definitions of exact replicas of a glob.

When loading a policy for a glob defintion, we choose the glob that has
the closest match to the namespace we are resolving for. We define the
closest match as the one with the _smallest character difference_
between the glob and the namespace we are matching.