backup: Do not copy UNIX-specific information

The `-a` switch to `rsync` causes it to copy extra information that may be
incompatible with the destination if it uses a non-native filesystem (such as
FAT or NTFS). Since most of this additional information is not necessary in a
backup and can cause copy errors, it should be excluded by default. Instead,
the `-r` flag is used to enable recursive copies, and the `-t` flag to
preserve file timestamps.

backup.py

@@ -15,7 +15,7 @@ class BackupError(Exception):
 class Backup(object):
 
     RSYNC = os.environ.get('RSYNC', 'rsync')
-    RSYNC_DEFAULT_ARGS = ['-aO', '--partial', '--delete']
+    RSYNC_DEFAULT_ARGS = ['-rtO', '--partial', '--delete']
     RSYNC_EXTRA_ARGS = shlex.split(os.environ.get('RSYNC_EXTRA_ARGS', ''))
 
     DEFAULT_CONFIG_FILENAME = 'backups.ini'