Use JS_ValueToECMAUint32 for File position, date, and length properties

This resolves errors when setting these properties to values > 2147483647
example:
!JavaScript  /sbbs/exec/load/sauce_lib.js line 69: Error: can't convert
2430770157 to an integer

That means you can now seek around (set position) within files > 2GB, truncate
or extend a file > 2GB, or set a file's date to > Jan-19-2038.