http://lists.alioth.debian.org/pipermail/bash-completion-devel/2010-February/002556.html
This mimics the old behaviour where you could reuse `_known_hosts()' as a helper function and pass it `-a' or `-c'. NOTE: Using `_known_hosts' as a helper function and passing options to `_known_hosts' is deprecated: Use `_known_hosts_real' instead.