mkfs.nilfs2: erase first and second sectors unless boot sector exists

There is a filesystem which writes a super block on the first sector
of the device.  One of such filesystems is xfs.  Since the current
mkfs.nilfs2 does not erase the first and the second sectors, blkid can
detect that the partition is used with the old file system even after
mkfs.nilfs2 has formatted the device.

This fixes the issue by erasing the first and the second if boot
sector does not exist on the first sector.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: update AUTHORS file

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: v2.1.5 release

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: allow to make without libblkid

Distributions with old blkid library fail to configure
nilfs-utils-2.1.5.

This fixes the issue by making the use of libblkid optional.

Now, user can eliminate dependency to libblkid by specifying
"--without-blkid" option when running the configure script:

 $ ./configure --without-blkid
 $ make

By default, the blkid library is used to mkfs.nilfs2 safer.

Reported-by: Seiji Kihara <kihara.seiji@lab.ntt.co.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs-utils: remove redundant selinux library references

When --with-selinux option is specified, redundant reference
to selinux library is added commonly.

This minimizes the use of -lselinux.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>