I've been writing some code which involves syncing files (like
rsync) and it became apparent that under Linux I could read
modification times (os.Lstat) with nanosecond precision but
only write them with microsecond precision.  This difference
in precision is rather annoying when trying to discover
whether files need syncing or not!

I've patched syscall and os to increases the accuracy of of
os.Chtimes for Linux and Windows.  This involved exposing the
utimensat system call under Linux and a bit of extra code
under Windows.  I decided not to expose the "at" bit of the
system call as it is impossible to replicate under Windows, so
the patch adds syscall.Utimens() to all architectures along
with a ImplementsUtimens flag.

If the utimensat syscall isn't available (utimensat was added
to Linux in 2.6.22, Released, 8 July 2007) then it silently
falls back to the microsecond accuracy version it uses now.
The improved accuracy for Windows should be good for all
versions of Windows.

Unfortunately Darwin doesn't seem to have a utimensat system
call that I could find so I couldn't implement it there.  The
BSDs do, but since they share their syscall implementation
with Darwin I couldn't figure out how to define a syscall for
*BSD and not Darwin.  I've left this as a TODO in the code.

In the process I implemented the missing methods for Timespec
under Windows which I needed which just happened to round out
the Timespec API for all platforms!

------------------------------------------------------------

Test code: http://play.golang.org/p/1xnGuYOi4b

Linux Before (1000 ns precision)

$ ./utimetest.linux.before z
Setting mtime 1344937903123456789: 2012-08-14 10:51:43.123456789 +0100 BST
Reading mtime 1344937903123457000: 2012-08-14 10:51:43.123457 +0100 BST

Linux After (1 ns precision)

$ ./utimetest.linux.after z
Setting mtime 1344937903123456789: 2012-08-14 10:51:43.123456789 +0100 BST
Reading mtime 1344937903123456789: 2012-08-14 10:51:43.123456789 +0100 BST

Windows Before (1000 ns precision)

X:\>utimetest.windows.before.exe c:\Test.txt
Setting mtime 1344937903123456789: 2012-08-14 10:51:43.123456789 +0100 GMTDT
Reading mtime 1344937903123456000: 2012-08-14 10:51:43.123456 +0100 GMTDT

Windows After (100 ns precision)

X:\>utimetest.windows.after.exe c:\Test.txt
Setting mtime 1344937903123456789: 2012-08-14 10:51:43.123456789 +0100 GMTDT
Reading mtime 1344937903123456700: 2012-08-14 10:51:43.1234567 +0100 GMTDT

R=golang-dev, alex.brainman, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/6905057