S9 EXT  (sys:lseek integer1 integer2 integer3)  ==>  integer

Move the file pointer of the file descriptor INTEGER1 to the
position INTEGER2 from the origin INTEGER3. Valid origin values
include:

        sys:seek-set (0) = seek from the beginning of the file
        sys:seek-cur (1) = seek from the current position
        sys:seek-end (2) = seek from the end of the file

SYS:LSEEK returns the current file position after performing the
seek operation.

(sys:lseek fd 0 sys:seek-end) will position the file pointer at the
end of the file and return the size of the file. Data subsequently
written to FD will hence be appended to the corresponding file.

(let ((fd (sys:creat "tmpfile")))
  (sys:write fd "test")
  (sys:lseek fd 0 sys:seek-end))   ==>  4
