The backup format now includes a stream header. That header begins with
a magic string and version number, then includes a flag stating whether
the archive data is compressed, and then in the case of encrypted archives
states the password salt used during encryption key stretching.
When compression is used, everything following the header is run through
a standard zlib "deflate" compressor before being sent downstream.
Change-Id: Ica72753e4ef2c3d13e63b45e7722a00652940a55
7bdb096289