cgls: print error messages when --unit and --user-unit are used together

cgls: print error messages when --unit and --user-unit are used together

Mixing the `--unit` and `--user-unit` options will result in error messages.
During the parsing phase, only the `arg_show_unit` record of the last
occurrence of the option is used; all names are placed in the same `arg_names`,
thus mixing the two types of units in the query.

For example, `-u foo --user-unit bar` will also treat `foo` as a user unit and
query it in the user service.