iofunc support for ns-resolution timestamps

The iofunc_attr_t structure includes unsigned mtime_ns, atime_ns, and ctime_ns fields to accompany the mtime, atime, and ctime fields:

• These fields are included by default for 64-bit architectures, but you can include them for 32-bit compiles as well by defining IOFUNC_NS_TIMESTAMP_SUPPORT before including <sys/iofunc.h>.
• If these fields are included, IOFUNC_ATTR_NS_TIMESTAMPS is set in the attribute's flags.
• The iofunc_time_update() function updates the nanosecond-resolution fields as well as the second-resolution fields, if IOFUNC_ATTR_NS_TIMESTAMPS is set in the attribute's flags.