Instead of raising "keychains not yet implemented" whenever we have an
explicit node ID, only raise the error when the given node ID isn't
the local node.  This allows folks to use the more-general
explicit-node-ID form in scripts and such now, as long as they use the
local node name when calling those scripts.

Also add a test for this case, and update the comment for the
one-argument case to match the current syntax for extracting a
multihash name string.

License: MIT
Signed-off-by: W. Trevor King <wking@tremily.us>