Define the concept of unresolved QName

Define the concept of unresolved QName

YANG Xpaths have dynamic namespace resolution for the default
namespace: during parse time we can only bind qualified node
identifiers.

Unqualified node identifiers require an explicit default namespace
binding step, which happens between when the XPath is parsed and
when it is evaluated, when the context node is determined.

This is quite visible in the context of leafref path expressions:
- typedef expressions are bound to the leaf/leafset referencing them
- leaf expressions in groupings are bound only after they have been
  completely instantiated (i.e. 'uses' outside of grouping context)

Change-Id: I65341793dc520bcc36c45dec76a303080be115a1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>