kvs: user facing commands may need fine tuning for namespaces

[garlick]

As noted in #1432

$ flux kvs namespace-create foo
$ flux kvs put ns
$ flux kvs put ns:foo/a=42
$ flux kvs --namespace=foo dir
a = 42
$ flux kvs dir ns:foo
flux-kvs: ns:foo: Invalid argument  ### should that work?
$ flux kvs dir ns:foo/
flux-kvs: ns:foo/: Invalid argument  ### seems like that should work
$ flux kvs dir ns:foo/.  ### seems like both double dots should not be displayed
ns:foo/..a = 42
$ flux kvs ls ns:foo/.  
ns:foo/: Invalid argument  ### that should work
$ flux kvs link  ns:foo/. linktofoo
$ flux kvs dir -R linktofoo
linktofoo.a = 42

Some of this seems a bit odd.

We should decide how flux kvs dir, flux kvs ls, etc should present to the user with namespaces thrown into the mix.

[chu11]

Thinking about this, I think

flux kvs dir ns:foo/

and

flux kvs dir ns:foo/.

should do the same thing, but think

flux kvs dir ns:foo

should be an error, b/c its an improperly formatted namespace specifier.