sd-id128: make id128_read() or friends return -ENOPKG when the file contents is "uninitialized"
Then, this drops ID128_PLAIN_OR_UNINIT. Also, this renames
Id128Format -> Id128FormatFlag, and make it bitfield.
Fixes #25634.
(cherry picked from commit 057bf780e9d45480fbacdd3b060dbe37b37f9693)