Documentation: replace ~/.bup with $BUP_DIR in bup-index.md and bup-save.md.

When one is not using the default bupdir placement, then those files
aren't found inside ~/.bup, but wherever "-d /path.." or $BUP_DIR are
pointing to.

Thanks to krichter722@aol.de for mentioning this.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Catch errors when restoring Linux attrs into xfs or btrfs

Both xfs and btrfs turn out to return EOPNOTSUPP, rather than ENOTTY,
when we try to apply Linux attrs that they do not support.  This patch
improves matters in that it ensures we can restore into these file
systems without crashing, but does not quite address all issues yet:

- if any of the attributes we try to apply is not supported, none of
  them will be applied to the file at all
- the user will typically drown in error output; eg. for a backup
  originating from an ext4 file system, we can expect endless error
  output relating to the extents-flag not being supported on xfs/btrfs

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Set python executables to python2 instead of relying that python points
to a python2 executable.

Also see PEP394 [1].

[1] http://www.python.org/dev/peps/pep-0394/