Detect OS X broken /etc/zshenv and suggest rename

If `/etc/zshenv` that calls `path_helper` is found on the system, assume
OS X version pre-El Capitan and suggest that this file gets renamed to
`zprofile` so that it only gets sourced on login shells and doesn't mess
up PATH order on nested invocation of zsh.

zshrc

@@ -111,5 +111,18 @@ _load_settings() {
 }
 _load_settings "$HOME/.zsh/configs"
 
+# detect old OS X broken /etc/zshenv and suggest rename
+if grep -qw path_helper /etc/zshenv 2>/dev/null; then
+  cat <<MSG >&2
+${fg[red]}Warning:${reset_color} \`/etc/zshenv' configuration file on your system may cause unexpected
+PATH changes on subsequent invocations of the zsh shell. The solution is to
+rename the file to \`zprofile':
+  ${fg_bold[white]}sudo mv /etc/{zshenv,zprofile}${reset_color}
+
+(called from ${(%):-%N:%i})
+
+MSG
+fi
+
 # Local config
 [[ -f ~/.zshrc.local ]] && source ~/.zshrc.local