Fix NAME section of manpages zhack and fsck.zfs.

In Debian GNU/Linux a program called 'linitian' is used to make sure that
packages conforms to the Debian GNU/Linux packaging guide lines.
This fixes the problem reported as:

     W: zfsutils: manpage-has-bad-whatis-entry usr/share/man/man1/zhack.1.gz
     W: zfsutils: manpage-has-bad-whatis-entry usr/share/man/man8/fsck.zfs.8.gz

Not something that ZoL needs to addhere to, but every other man page have their
NAME section in a special way - why not these two as well?