reload postfix instead of restart on config change

reload postfix instead of restart on config change

From `man 1 postfix`:
```
Note: in order to refresh the Postfix mail system after a
configuration change, do not use the start and stop commands in
succession. Use the reload command instead.
```

Additionally restarting postfix, while a mail has been passed to
pmg-smtp-filter but has not beed fed back to postfix again, causes it
to get passed a second time to pmg-smtp-filter (by qmgr) and this
results in duplicate maildelivery

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>