[krbdev.mit.edu #6747] openssl k5crypto back end should not require openssl-1.0

[Tom Yu via RT]

Currently, the OpenSSL k5crypto back end requires the use of openssl-1.0, because earlier 
releases did not support CTS mode.  For portability reasons (and minor performance 
improvement in the IOV case), we should implement CTS mode using CBC mode interfaces that 
already exist in older OpenSSL releases.