homed maintains two or three copies of the user's identity record per
home directory: one on the host, one inside the LUKS header, and one
embedded in the home directory.
Previously we'd insist that if a user logs in they have to authenticate
against all three, as a safety feature. This broke logging into
unfixated records however, since in that case the host version is
synthetic and thus does not carry any authentication data.
Let's hence losen the strictness here: accept authentication against
host records that carry no auth data. This should be safe as we know
after all that the second/third record will catch invalid accesses.
Fixes: #15178
Let's make the logic a bit smarter: if we detect that /home is
encrypted, let's avoid double encryption and prefer plain
directory/subvolumes instead of our regular luks images.
Also, allow configuration go storage/file system via an env var passed
to homework. In a later commit, let's then change homed to initialize
that env var from a config file setting, when invoking homework.
Make use of the new user_record_build_image_path() helper the previous
commit added to share some code.
Also, let's make sure we update all parsed-out fields with the new data
from the binding, so that the parsed-out fields are definitely
up-to-date.