Make mounted partition usage method selectable per file system (#683255)

Each file system class can now choose how the size and free space of the
file system is determined when it is mounted.

    .fs.online_read = FS::NONE  (default)
        Do nothing.  Don't get the file system size and free space.

    .fs.online_read = FS::GPARTED
        Use internal GParted method which calls statvfs() system call on
        the mounted file system.

    .fs.online_read = FS::EXTERNAL
        Call the file system's member function set_used_sectors().  This
        is the same function as called when the file system is not
        mounted.   It can determine if the file system is mounted or not
        by testing partition.busy and acting accordingly.

This means that determining the size and free space of active LVM2
Physical Volumes is no longer a special case.  Instead the lvm2_pv class
just elects to have its set_used_sectors() method called for both the
active and deactive cases.

Bug #683255 - ext2: statvfs differs from dumpe2fs (x MB unallocated
              space within the partition)