bash-completion: use the first argument instead of the global variable (#6457)

Without this fix:

$ systemctl start <tab>
Display all 135 possibilities? (y or n)
$ __get_startable_units --system | wc -l
224

the number of the suggestions are quite different, as __get_startable_units --system does
not filter already started units. With this fix,

$ systemctl start <tab>
Display all 135 possibilities? (y or n)
$ __get_startable_units --system | wc -l
123
$ __get_template_names --system | wc -l
12

the number of the suggestions matches one the function returns.
For consistency with the other internal functions, it should use the first argument
instead of the global variable $mode.

[zj: add commit message to make it sound like we know what we're doing]