Using non-UFS or volume root filesystems

Make the system capable to mount root filesystems other than UFS -- e.g. using ZFS as root filesystem.

This implies separating the kernel and bootstrap loader on a small /boot partition and be able to convince the kernel to mount the right volume/filesystem.

Things to take into consideration:
 * keep the partition as "a" magic slice (boot slice), its contents being mounted under /boot
 * that implies creating a self link to keep the loader happy (boot -> .)
 * the "boot" symlink requires renaming the "/boot/boot" boostrap code file
 * the boot0 MBR loader should try first paths with shortened format (/loader and /kernel/kernel)
   * check that kern.bootfile sysctl value matches upon bootup
 * vfs.root.mountfrom should be defined in /boot/loader.conf pointing to the right fs type and mountpoint (e.g. "zfs:sysvol")