systemctl: switch-root - read init= from /proc/cmdline

If the path to init is not specified as an argumnt to systemctl, but
init= is given on the kernel commandline, use that.

This means the initrd does not need glue code to parse the kernel
commandline before passing on init= to systemctl.

Cc: Harald Hoyer <harald@redhat.com>
Cc: Dave Reisner <dreisner@archlinux.org>