fstab-generator: generate new_root.mount in initrd

fstab-generator: generate new_root.mount in initrd

The configuration is taken from /proc/cmdline, aiming at emulating the
behavior of the kernel when no initramfs is used.

The supported options are: root=, rootfstype=, rootwait=, rootflags=,
ro, and rw. rootdelay= was dropped, as it is not really useful in a
systemd world, but could easily be added.

v2: fix comments by Lennart, and complain loudly if root= can not be found

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