Replace os.*stat() with xstat.*stat(); use integer ns for all fs times.

Replace os.*stat() with xstat.*stat(); use integer ns for all fs times.

Replace all calls of the os.*stat() functions with calls to the xstat
equivalents.  This should leave bup with the xstat stat representation
(and integer ns timestamps) everywhere.

Remove FSTime, and add a few xstat conversion functions to replace the
bits we still want: timespec_to_nsecs(), nsecs_to_timespec(),
fstime_floor_secs(), fstime_to_timespec().

Signed-off-by: Rob Browning <rlb@defaultvalue.org>