Add new assertions to verify atime, birthtime, and mtime, both

Add new assertions to verify atime, birthtime, and mtime, both
for exact values and "recent".  Sketch in support for FreeBSD
(including high-res), Windows (including high-res and birthtime),
and generic POSIX.

Inspired in part by the discovery that stat() on Windows shifts mtime
values depending on DST settings, so the old code that used stat()
breaks in weird ways on Windows.  GetFileTime() seems to not have this
problem, but using that requires that we not use stat().

SVN-Revision: 1431