Use pax extended tar format to support long filenames etc.

'tar' supports only 100-character paths; 'ustar' supports only
155+100 character prefix + paths; neither supports files larger
than about 8 gigabytes.  We now use the POSIX.1-2001 'pax'
extended tar format for those files in the backup stream that
are too large or have too-long paths for the 'ustar' format.

Change-Id: I2f256823091deaec9b1ccea685d2344753c6cb67