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)