Support more accurate timestamps for utimens().

Support more accurate timestamps for utimens().

Added an option to Fuse::main(), specifically "utimens_as_array",
that causes timestamps to be passed as arrays containing the UNIX
timestamp (seconds since the epoch), followed by the number of
nanoseconds, instead of as a floating-point value, for better
precision. However, I've noticed that when (on Linux, at least)
you "touch" a file, which ends up with utimensat() getting passed
a NULL value for the timestamp array, the resulting timestamp that
utimens() gets always has a "0" for the nanosecond field. I think
that's a FUSE misfunction.