This changes the defaults:
- AUTOENV_FILE_ENTER: .env => .autoenv.zsh
- AUTOENV_FILE_LEAVE: .env_leave => .autoenv_leave.zsh
`.env` is usually used only for key-value pairs for environment
settings, e.g. with foreman. We do not want to interfere with this.
This also renames the setting/variable AUTOENV_ENV_FILENAME to
AUTOENV_AUTH_FILE, and uses the [XDG
spec](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables)
for the default location.
The new default location for the auth file will be
~/.local/share/autoenv_auth.
When the old setting is used, or the previous default exists, a warning
is given with instructions.