Bundle 9p modules in an initramfs

This makes --kvm work with stock Ubuntu kernels.

Don't create an initramfs with an explicit --kernel
argument; custom kernels must have 9p built-in.

Also mount devtmpfs early in the initramfs case.
watchdog: modprobe if the watchdog isn't built-in
README: document distribution kernel support
Documentation: Rework the README and the --help