list: handle non-symlink dirs and URLs in POW_PATH

Changes helper method get_app_origin to conform much closer to the logic
used by pow itself.

In the process, introduces reporting-support for URL proxy files,
removes crashes on encountering non-symlink dirs, and ignores extraneous
files (e.g. badly formatted proxying files or, if they somehow appear, unix
sockets, device files, etc.)
list: report on symlinked public dirs, Fixes #84