(#8740) Do not enumerate files in the root directory.

Previously the command 'puppet resource file' would enumerate all files
in the root directory, and generate an exception if the file type was
not a directory, file, or link. Worse, it would also do this when a file
or directory was specified, e.g. 'puppet resource file /etc/hosts'.

Ideally, the find method of the ral terminus should not need to call the
type's instances class method, instead just creating an instance of the
type with the specified name and parameters. However, some types, like
package, depend on this behavior. The type walks all providers and all
instances that they provide, checking to see if the provider provides an
instance with that name, and also warning if another provider provides
an instance with the same name.

Also, ideally, puppet should not blow up when encountering an
unsupported file type, e.g. Unix domain socket, but that would be too
big of a change for 2.6.x.

This commit changes 'puppet resource file' to return a message saying
that the operation is not supported:

Listing all file instances is not supported.  Please specify a file
or directory, e.g. puppet resource file /etc

The change is bit of a hack, as ideally, the file type's instances
method could raise an exception when called in a 'search' context, but
return an empty array in a 'find' context. But that also would be too
big of a change for 2.6.x.

This commit also adds spec tests for the resource application and file
type, as well as an acceptance test, which creates a Unix domain socket
in the root directory, while running 'puppet resource file'.

Paired-with: Nick Lewis <nick@puppetlabs.com>
Reviewed-by: Jacob Helwig <jacob@puppetlabs.com>