shutdown: pass own argv to /run/initramfs/shutdown

Since commit b1e90ec515408aec2702522f6f68c4920b56375b systemd passes
its log settings to systemd-shutdown via command line parameters.
However, systemd-shutdown doesn't pass these parameters to
/run/initramfs/shutdown, causing it to fall back to the default log
settings.

This fixes the following bugs about the shutdown not being quiet
despite "quiet" being in the kernel parameters:

https://bugs.freedesktop.org/show_bug.cgi?id=79582
https://bugs.freedesktop.org/show_bug.cgi?id=57216