feat(fatfs): remember by which core an FD was opened, allow to close all FDs by a certain core

untested as of right now, but we can and do now close all FDs opened by core1 in hardware_reset(). In a fatfs_detach, we close all just like before.

Whether an FD is owned by core1 is determined via the API_CALL_MAM->call_flag member, since an API call is the only way for obtain an FD at all.