Two changes:

  * 'getModTime' now uses 'modificationTimeHiRes' instead of 'modificationTime'
    on Unix when the former is available.

  * 'ModTime' is now represented as a 64-bit unsigned integer in Windows UTC
    format (that is, 100 ns resolution and day zero is 1601-01-01) on all
    platforms. Previously we used POSIX seconds, which was wrong (low
    resolution). Sandbox timestamp files in old format are now up-converted on
    the fly.

Fixes #3132.