Add non-GPL ext2 filesystem (port from FreeBSD)

The new ext2 will replace the existing sys/gnu/vfs/ext2fs in future,
which is known to have lots of issues. This is already better than
sys/gnu/vfs/ex

Add non-GPL ext2 filesystem (port from FreeBSD)

The new ext2 will replace the existing sys/gnu/vfs/ext2fs in future,
which is known to have lots of issues. This is already better than
sys/gnu/vfs/ext2fs in many ways, but will keep it as an option for now
as there seems to be htree dirents and/or vop_readdir related bugs.

The new implementation supports ext3 htree dirents. Read support for
ext4 extents which exists in FreeBSD ext2 isn't implemented yet (TODO).
ACL/xattr are unsupported, and likely never will be.

With exception of DragonFly VFS specific part that can't reuse FreeBSD
code (vfsops, many vnops, bread/getblk/bio i/f difference, etc),
the new ext2 is designed to be close to upstream as much as possible.
The code is there for a reason to make clear the *real* diff without
introducing nonfunctional diff noises.

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