sbin/newfs_hammer: Add hidden -j option for header junk space

sbin/newfs_hammer: Add hidden -j option for header junk space

There is 256KB header junk space (which includes volume header)
starting from offset 0 of the filesystem partition up until
where boot_area/memory_log starts, but the header junk space
beyond volume header isn't used for anything.

This commit adds -j option to specify header junk size smaller
than 256KB. -j option does exactly the same as how -b,-m,-u
options allow below low limit when -f is specified.

Don't mention this option in newfs_hammer(8) because there is
no reason for users to use this. The header junk space exists
only for future enhancement of volume header (though most likely
will never be used), so users should just stick with 256KB.