Improve the 'getModTime' implementation.
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.