nodogsplash: fix config generation in the init script

nodogsplash: fix config generation in the init script

Three problems in the generated configuration:

- The error message for a missing configuration file prints $file,
  which is never set, instead of $val, which holds the path.
- The warning for the unsupported FAS options prints the value of
  the option instead of its name, so the message does not say which
  option is unsupported.
- gatewayinterface is emitted twice: once resolved to a device via
  network_get_device() as "GatewayInterface", and once more by the
  generic option loop with the raw UCI value. When the value names a
  network section rather than a device, the second line overrides
  the resolved device with a name nodogsplash cannot use.

Reported-by: openwrt-ai[bot]
Signed-off-by: Josef Schlehofer <redacted>
Co-authored-by: Claude Fable 5 <redacted>