tail -F: don't stop following the target of a rename

tail -F: don't stop following the target of a rename

This fixes a bug whereby tail -F would fail to track changes
to a file that was a target of a rename, and when the source of
the rename was another tailed file.

* src/tail.c (tail_forever_inotify): Ensure the wd is not already
present in the hash table before trying to add it.  When a new watch
descriptor is added to the `wd_to_name' hash table, check that it is
not already present.  If it is present then remove the previous element.