fsck.gfs2: Make -p, -n and -y conflicting options
    
    Exit with FSCK_USAGE when these options are used in combination. Also
    update the man page to make this behaviour explicit with other tweaks
    for conciseness. Tests included.
    
    Resolves: rhbz#1507091
    
    Signed-off-by: Andrew Price <anprice@redhat.com>