shutdown: fix arguments to /run/initramfs/shutdown

Our initrd interface specifies that the verb is in argv[1].
This is where systemd passes it to systemd-shutdown, but getopt
permutes argv[]. This confuses dracut's shutdown script:
  Shutdown called with argument '--log-level'. Rebooting!

getopt can be convinced to not permute argv[] by having '-' as the first
character of optstring. Let's use it. This requires changing the way
non-option arguments (in our case, the verb) are processed.

This fixes a bug where the system would reboot instead of powering off.