dbus: Fixes Bug#13094 - Check for existing user before `useradd`

dbus: Fixes Bug#13094 - Check for existing user before `useradd`

- The dbus install.sh script useradd command causes an error:
   "failed adding user 'messagebus', exit code: 9"
- This patch adds a check to only do the useradd if the user does not exist.
- See the bump PAK_VER for dbus that Adolf publised.  See this patch:
https://lists.ipfire.org/pipermail/development/2023-April/015816.html

Signed-off-by: Jon Murphy <jon.murphy@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>