libudev: add missing errno initialization/error propagation (#6781)

libudev: add missing errno initialization/error propagation (#6781)

In libudev (which es much older code than the rest of systemd), we
propagate errors in functions not returning an int, via (positive) errno
(i.e. libc-style), and as negative Exyz values in those returning an int
(much preferred, i.e. Linux kernel style). Let's fix up a few place,
where this was incorrectly done, or not done at all.

Fixes: #6613