Detect if ~/.zshenv.local changes PATH and suggest .zshrc instead

Warn people who might have PATH and similar configuration in their
`~/.zshenv.local` that they should upgrade to `~/.zshrc.local` since
that's a much better phase for such configuration. This is for backwards
compatibility with people's personal configurations from pre-El Capitan
days.

The generic `.zshenv` file from zsh distribution[1] advises:

> .zshenv is sourced on ALL invocations of the shell, unless the -f
> option is set. It should NOT normally contain commands to set the
> command search path, or other common environment variables unless you
> really know what you're doing. E.g. running `PATH=/custom/path gdb program`
> sources this file (when gdb runs the program via $SHELL), so you want
> to be sure not to override a custom environment in such cases. Note
> also that .zshenv should not contain commands that produce output or
> assume the shell is attached to a tty.

[1]: http://sourceforge.net/p/zsh/code/ci/master/tree/StartupFiles/zshenv

zshenv

@@ -1,2 +1,23 @@
+local _old_path="$PATH"
+
 # Local config
 [[ -f ~/.zshenv.local ]] && source ~/.zshenv.local
+
+if [[ $PATH != $_old_path ]]; then
+  # `colors` isn't initialized yet, so define a few manually
+  typeset -AHg fg fg_bold
+  fg[red]=$'\e[31m'
+  fg_bold[white]=$'\e[1;37m'
+  reset_color=$'\e[m'
+
+  cat <<MSG >&2
+${fg[red]}Warning:${reset_color} your \`~/.zshenv.local' configuration seems to edit PATH entries.
+Please move that configuration to \`.zshrc.local' like so:
+  ${fg_bold[white]}cat ~/.zshenv.local >> ~/.zshrc.local && rm ~/.zshenv.local${reset_color}
+
+(called from ${(%):-%N:%i})
+
+MSG
+fi
+
+unset _old_path