hwdb: port to flink_tmpfile()

hwdb: port to flink_tmpfile()

And modernize heavily while doing so.

Fixes: #21787

(Strictly speaking, this leaves a race window open: the the system is
powered off in the short interval when we linked in the prepared hwdb
file into the dir under a temporary name and are about to rename it to
the final name, then the file might be left over after all. But this
minimizes the window so much that this shouldn't be an issue in
real-life. Key after all is that with this change we'll build up the
hwdb file under O_TMPFILE, and thus are robust to power loss during the
slow operation)