Remember path (or other description) in fd's, add getter

Remember path (or other description) in fd's, add getter

- If opened by path, we obviously want to use that. Otherwise
  generate a description lazily on first Fdescr() call: on Linux
  we can grab something relatively meaningful by looking up from /proc
  (this is why we want to be lazy here...). If that's not available
  or fails, just generate a string on the current fdno. Actual
  paths are returned as is, other descriptions are bracketed,
  (eg "[mumble 123]").
- This makes it possible to give more meaningful error messages in
  places where we only get an fd from somebody (related to RhBug:522160)