unbound-dhcp-leases-bridge: Implement atomic file replacement

unbound-dhcp-leases-bridge: Implement atomic file replacement

This change no longer renames the file, but removes the old link and
creates a new link for the temporary file. That helps us to jump out of
the code at any point without worrying about cleaning up the temporary
file.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>