lib: Fix switched arguments in File.seek

[obiwac]

WhenceA and OffsetI arguments passed to OS.lSeek from File.seek are the wrong way around:

{OS.lSeek D O case W
              of 'set'     then 'SEEK_SET'
              [] 'current' then 'SEEK_CUR'
              [] 'end'     then 'SEEK_END'
              end _}

As per http://mozart2.org/mozart-v1/doc-1.4.0/system/node56.html, the offset should be passed after WhenceA.