fdisk: add a warning and truncate disks with >= 2^32 sectors

fdisk: add a warning and truncate disks with >= 2^32 sectors

As a result, for sectors we can use uint32_t instead of long long,
and on 32 bits it has drastic effects:

function                                             old     new   delta
get_geometry                                         619     646     +27
set_sun_partition                                    148     150      +2
get_partition                                        134     135      +1
xbsd_write_bootstrap                                 382     381      -1
xbsd_readlabel                                       247     246      -1
bsd_select                                          1674    1672      -2
sun_other_endian                                       4       1      -3
scsi_disk                                              4       1      -3
floppy                                                 4       1      -3
fdisk_main                                          3735    3732      -3
read_maybe_empty                                      43      37      -6
create_doslabel                                      111     104      -7
read_line                                             97      88      -9
add_logical                                          117     107     -10
write_table                                          599     588     -11
new_partition                                       1684    1670     -14
list_disk_geometry                                   229     215     -14
wrong_p_order                                        130     110     -20
xselect                                             3142    3114     -28
seek_sector                                           71      40     -31
get_boot                                            1576    1533     -43
fill_bounds                                          174     128     -46
delete_partition                                     603     551     -52
list_table                                          1401    1232    -169
set_partition                                        459     286    -173
verify                                              1840    1495    -345
add_partition                                       2486    1270   -1216
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/24 up/down: 30/-2210)       Total: -2180 bytes
   text    data     bss     dec     hex filename
 848812     460    7116  856388   d1144 busybox_old
 846620     460    7108  854188   d08ac busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>