Working for ext4 support

I had gotten some requests to support brand-new ext4 filesystem from my backport d-i users. Yesterday I implemented it. Download via previous article.

The work was harder than I assumed...

• The implementation on d-i side was mostly done already on d-i trunk by cjwatson and otavio. I appreciate their good job! I merged it to partman-base and partman-ext3.
• Because GNU parted in Lenny didn't support ext4, I had to backport from unstable. Plus, I had to build again some packages to follow ABI change.
• e2fsprogs-udeb in Lenny didn't contain mkfs.ext4, though deb contained it. I backported from unstable for stability.
• grub won't work with ext4. You have to use grub-pc instead of it, if you'd like to use ext4 for root (boot) partition. I left this issue alone and just wrote down FAQ.
• Even you choose grub-pc, it won't boot because initramfs misrecognizes ext4 partition as ext3. You have to append "rootfstype=ext4" option to kernel parameter. I wrote it on FAQ.