main: initialize default unit little later (#4321)

systemd fills arg_default_unit during startup with default.target
value. But arg_default_unit may be overwritten in parse_argv() or
parse_proc_cmdline_item().

Let's check value of arg_default_unit after calls of parse_argv()
and parse_proc_cmdline_item() and fill it with default.target if
it wasn't filled before. In this way we will not spend unnecessary
time to for filling arg_default_unit with default.target.