Suppport non-ECB cipher modes that require an IV.

This allows the cipher to be set to things like "Blowfish/CBC/PKCS5Padding", or to other chaining modes that require an IV.

Note that the default of using Blowfish (or any block cipher for that matter) in ECB mode is quite insecure; I would _highly_ recommend the default be changed to "Blowfish/CBC/PKCS5Padding" or similar. However, it would break existing installations that A) don't specify the cipher algorithm explicitly (i.e. rely on the default), and B) have cookies that are expected to survive an update - these would have to make sure to specify the algorithm now.