spaceman: Add table header for -d option

spaceman: Add table header for -d option

The xfs_spaceman utility did not have a a proper title to the content
displayed using -d (debug) option.

Old output:

$ sudo xfs_spaceman -c "freesp -d" /media/xfs
       0        4        1
       0        5        1
       0        6        1
       0        7        1
.....

This patch adds a suitable title header to the output, making it much
more readable to the user.

New output:

$ sudo xfs_spaceman -c "freesp -d" /media/xfs
    agno    agbno      len
       0        4        1
       0        5        1
       0        6        1
       0        7        1
....

Signed-off-by: Dhruvesh Rathore <dhruvesh_r@outlook.com>
Signed-off-by: Amey Ruikar <ameyruikar@yahoo.com>
Signed-off-by: Somdeep Dey <somdeepdey10@gmail.com>