Handle kernel/FUSE disagreement over Linux attrs

Use long for the Linux attr type, so that it should work on all
little-endian systems, across both normal and FUSE-backed filesystems.

Disable Linux attr support for now on big-endian systems where
sizeof(long) > sizeof(int).  See the changes to bup-index.md and
_helpers.c for more information.

Eventually, we may want to add an argument to allow re-enabling
support on affected systems, when the user can guarantee a homogeneous
filesystem type -- or better yet, the kernel and FUSE will finally
sort out their issues.

Thanks to Tilo Schwarz <mail@tilo-schwarz.de> and daryl5@arcor.de for
reporting and pursuing the problems that lead to this patch.

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