fuse/impl_{a,b}: improve node-url handling

fuse/impl_{a,b}: improve node-url handling

specifically change the expectation of the code to be such that the node-url
(self.url) always includes the trailing slash to be a correctly formed url

moreover read the node-url from the 'node.url' file found in the node 'basedir'
and only if that doesn't exist, then fall back to reading the 'webport' file
from therein and assuming localhost.  This then supports the general tahoe
pattern that tools needing only a webapi server can be pointed at a directory
containing the node.url file, which can optionally point to another server,
rather than requiring a complete node dir and locally running node instance.