Add script for generating zcrypt keys

[adehnert@mit.edu]

Creating a zcrypt key has some pitfalls for the unwary user: in particular, zcrypt will stop reading a key at a null byte or newline (AIUI). It's also not totally obvious how long a key should be. We should have a script in the source tree and the locker for easily generating zcrypt keys of the proper length and character set, so people don't accidentally generate week keys.