Use UtimesNanoAt from golang.org/x/sys/unix instead of manually crafting
the syscall.
Since UtimesNanoAt is provided for all unix-like OSes, factor out
lchtimes to its own file with appropriate build tags. This allows to
make use of it on darwin, dragonfly, freebsd, openbsd, netbsd and
solaris in addition to linux.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
