kvs: Handle keys with "." at the end.

[chu11]

As discussed in #1172 , flux should be able to handle keys with periods at the end. For example:

>flux kvs ls -R a

works but

>flux kvs ls -R a.
a.: No such file or directory

Does not. This behavior should be consistent with other kvs commands, even those that don't work with directories. For example with kvs get.

>flux kvs get resource.
flux-kvs: resource.: No such file or directory
>flux kvs get resource
flux-kvs: resource: Is a directory

This should return the same error code, not different ones.

Unclear if this handling would be solely within the flux kvs module or if some handling may come with the flux kvs command and/or libkvs.