Use lseek64 and ltell64 to support large object greater than 2gb in size...

If a large object is greater than 2gb then calls to seek and tell will fail with an OperationError. This is because of the use of lseek and ltell instead of the lseek64 and ltell64.

Replaces lseek and ltell with lseek64 and ltell64 respectively. lobject_seek and lobject_tell modified to use long instead of int.