Use /etc/hosts in addtion to $HOME/.ssh/known_hosts as the source for
hostname completion
Turn on completion caching to speed up certain comands
When completing usernames, don't include system accounts by default
The key here is not using MENU_COMPLETE which autoselects the first
entry, but using AUTO_MENU which displays the menu but autoselects
on successive tab hit.
Also, do not use zstyle menu yes because that somehow overrides
the setopts.