unix_listener_stats follows and remembers symlinks

Teach unix_listener_stats to remember the symlink path
it followed and have it point to the same object as the
resolved (real) socket path.

This allows the case where looking up stats by symlinks
works if the symlink is given to unix_listener_stats:

  File.symlink("/real/path/of.sock", "/path/to/link.sock")
  stats = unix_listener_stats(["/path/to/link.sock"])
  stats["/path/to/link.sock"] => # same as stats["/real/path/of.sock"]

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