This commit implements the blockMACBytes and blockMACRandBytes options
that cause a header to be inserted in front of every file block
containing a MAC of the block (and optionally some random data).
Added support to EncFSConfig, EncFSConfigParser and EncFSConfigWriter
to recognize these options. Implemented the MAC computation,
verification and insertion in EncFSInputStream and EncFSOutputStream.
Added a new test volume configured with 8 bytes of MAC and 8 bytes of
random bytes as 'testvol-blockmac', and extended EncFSVolumeTest to
include a long file test that verifies the MAC computations.
Mark Pariente