ukify: avoid treating invalid option as an argument

ukify: avoid treating invalid option as an argument

For some reason, argparse treats undefined options as positional args in
certain scenarios:

$ src/ukify/ukify.py --badopt='11'
ukify.py: error: unrecognized arguments: --badopt=11
$ src/ukify/ukify.py --badopt '11'
ukify.py: error: unrecognized arguments: --badopt
$ src/ukify/ukify.py --badopt '11 12'
Assuming obsolete command line syntax with no verb. Please use 'build'.
Traceback (most recent call last):
  File "/home/zbyszek/src/systemd/src/ukify/ukify.py", line 2497, in <module>
    main()
    ~~~~^^
  File "/home/zbyszek/src/systemd/src/ukify/ukify.py", line 2485, in main
    check_inputs(opts)
    ~~~~~~~~~~~~^^^^^^
  File "/home/zbyszek/src/systemd/src/ukify/ukify.py", line 671, in check_inputs
    value.open().close()
    ~~~~~~~~~~^^
  File "/usr/lib64/python3.13/pathlib/_local.py", line 537, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '--badopt=11 12'

I suspect that this is some crap compat for Windows, where option parsing is
an even bigger mess than here.

Being told about positional args, when no positional args were specified is
confusing, so add a check for this.