Drop xstat floating point timestamp support -- use integer ns.

Drop xstat floating point timestamp support -- use integer ns.

Drop conditional support for floating point timestamps (the xstat
os.stat() fallback).  Switch to integer nanosecond timestamps
everywhere except the metadata record encoding and _helpers.c.

The metadata encoding is still a timespec because separate s and ns
timespec vints compress much better, and timespecs are still returned
by _helpers because the conversion to integer nanoseconds is much more
convenient in Python.

Enforce timespec range expectations -- throw an exception if the
system returns a nanosecond value less than 0 or greater than
999999999.  Remove _have_ns_fs_timestamps.

Depend on bup_stat(), bup_stat(), and bup_lstat() unconditionally, and
change the timespec return conversion from "(ll)" to "(Ll)", i.e. long
long range for secs.

This commit may break the build on some platforms -- we'll have to add
suitable conditionals once we see what's needed.

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