zed.d/all-debug.sh: simplify

By locking the log file itself, we can omit arduous rebinding and
explicit umask setting, but, perhaps more importantly, avoid permanently
littering /var/lock/ with zed.debug.log.lock we will never delete

It is imperative that the previous commit
("zed-functions.sh: zed_lock(): don't truncate lock")
be included in any series that contains this one

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12042