Add support for running fsck

- Add empty /etc/fstab and symlink /etc/mtab to /proc/mounts;
  not essential, but quell a number of fsck warnings
- Copy fsck and needed fsck helpers, plus logsave
- Add checkfs function, based on the initscripts checkroot
  script
- local mount functions will call checkfs prior to mounting
  the filesystem

Rebased and dropped ETC support by Michael Prokop <mika@debian.org>