src: fix max file limits in systemd services

src: fix max file limits in systemd services

This fixes

  commit 38abf9c34dc481b0dc923bdab446ee623bdc5ab6
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Jun 21 13:22:40 2023 +0100

    src: set max open file limit to match systemd >= 240 defaults

The bug referenced in that commit had suggested to set

  LimitNOFile=512000:1024

on the basis that matches current systemd default behaviour and is
compatible with old systemd. That was good except

 * The setting is LimitNOFILE and these are case sensitive
 * The hard and soft limits were inverted - soft must come
   first and so it would have been ignored even if the
   setting name was correct.
 * The default hard limit is 524288 not 512000

Reported-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>