core.app: fix a bug related to getting app zone name
core.app: fix a bug related to getting app zone name

If running an app defined in a script via snabb snsh,
getfenv(app.start).__NAME would be undefined and trigger our error handler
for undefined variables.

Use rawget to avoid this, and properly fallback to the app name.

snabbco/snabb,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,eugeneia/snabb,snabbco/snabb,eugeneia/snabb,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,snabbco/snabb,snabbco/snabb