zsh_completion: Move helper function to autoload

_hosts_or_user_at_host was used by 6 different completions, and
previously was in all 6 of those files. I moved it out to its own file,
_sd_hosts_or_user_at_host. This will be autoloaded for use in other
completion functions. It also allows external completions to use this
function by simply calling _sd_hosts_or_user_at_host as in the systemd
completions.

shell-completion/zsh/_hostnamectl

@@ -1,11 +1,5 @@
 #compdef hostnamectl
 
-_hosts_or_user_at_host() {
-  _alternative \
-    'users-hosts:: _user_at_host' \
-    'hosts:: _hosts'
-}
-
 _hostnamectl_command() {
     local -a _hostnamectl_cmds
     _hostnamectl_cmds=(
@@ -34,5 +28,5 @@ _arguments -s \
     '--static[Only set static hostname]' \
     '--pretty[Only set pretty hostname]' \
     '--no-ask-password[Do not prompt for password]' \
-    {-H,--host=}'[Operate on remote host]:userathost:_hosts_or_user_at_host' \
+    {-H,--host=}'[Operate on remote host]:userathost:_sd_hosts_or_user_at_host' \
     '*::hostnamectl commands:_hostnamectl_command'