bootctl: warn if the ESP random seed is stored on a world-readable dir

bootctl: warn if the ESP random seed is stored on a world-readable dir

This takes heavy inspiration from @zx2c4 (Jason A. Donenfeld)'s
PR #25531 but changes it considerably, but always going by fd instead of
paths, and only warning about the side file itself and the ESP mount
point, nothing else. This shuld be more than enough and should not be
brittle against concurrent path modifications.

Replaces: #25531