WebUI: use `filepatharg` in `onload` path.

Fixes #3296.
DVB: Fix accidentally removed line in
594935d8d533cb30d7cfcc7d186dee50a54c4af1 "Split PM_CAPTURE mode into
PM_ANALOG_CAPTURE and PM_DIGITAL_CAPTURE"

This might cause DVB mode to lock in certain cases.

Fixes #4178