The problem was that os.Chtimes() operates on the target of the
symlink and not on the symlink itself.
When tar extracts an archive containing a symlink, it actually wants
to set the times on the symlink. This can also be tested using
"touch -h".

This patch adds a wrapper for the Linux utimensat(2) syscall
that allows passing flags along with a unit test.

It uses the flag AT_SYMLINK_NOFOLLOW to implement
loopbackFileSystem.Utimens() properly.

It also add handling for UTIME_NOW and UTIME_OMIT that was already
present in loopbackFile.Utimens() but missing in loopbackFileSystem.

Fixes issue 81 ( https://github.com/hanwen/go-fuse/issues/81 ).