gss_krb5: Use random value to initialize confounder

Initialize the value used for the confounder to a random value
rather than starting from zero.
Allow for confounders of length 8 or 16 (which will be needed for AES).

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>