Magic Folder: the localdir argument to the 'tahoe magic-folder {create,join}' commands should be expanded to an absolute path

[daira]

If you pass a relative path as the local directory to the tahoe magic-folder create or tahoe magic-folder join command, it will be copied into the node's tahoe.cfg entry for [magic-folder]local.directory as-is.

This causes the running node to expand the path relative to its node directory, which is counterintuitive and insecure (since the local directory tree will include the private/ subdirectory).

The create and join commands should expand the path relative to the current directory of the command. It may also be useful to detect cases where there is a node directory underneath a local directory and avoid uploading those files, in case the user does this by accident. (This might cause an infinite regress if the node directory is for a server that is storing shares for the Magic Folder DMD -- and is in any case undesirable.)