[recovery-zip] Autodetect fstab column order and support flashing to e.g. userdata

Reopen of pmbootstrap!1561 (closed)

Suggestions from the original MR were applied except for https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1561/diffs?w=0&view=parallel#note_85239722

Before, flashing was just possible to system / external_sd and not to userdata ("data").