zed.8: don't pretend an unprivileged user could change the script owner

And add a note on /why/ ZEDLETs need to be owned by root

Quoth chown(2), Linux man-pages project:
  Only a privileged process (Linux: one with the CAP_CHOWN capability)
  may change the owner of a file.

Quoth chown(2), FreeBSD:
     [EPERM]  The operation would change the ownership,
              but the effective user ID is not the super-user.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11834

cmd/zed/zed_conf.c

@@ -324,8 +324,6 @@ zed_conf_parse_opts(struct zed_conf *zcp, int argc, char **argv)
  *
  * Return 0 on success with an updated set of zedlets,
  * or -1 on error with errno set.
- *
- * FIXME: Check if zedlet_dir and all parent dirs are secure.
  */
 int
 zed_conf_scan_dir(struct zed_conf *zcp)