- also fix issue in which an empty
scheme was a valid URI.
- and rename http_link_pattern to uri_link_pattern
Since the pattern does actually match more than just http-scheme
URIs it is renamed to reflect the content more precisely.
For autochdir compatibility it will first try to resolve the path
relative to the current namespace; if this fails it will default
back to a path relative to the wikis root directory.
Also add a helper function to determine full namespace path.