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

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