Warn on weak zcrypt keys

[adehnert@mit.edu]

Given the difficulties in #221 and a recently-discovered weak key, I suspect there are a bunch of weak zcrypt keys. We should supply something to help detect and fix weak zcrypt keys. Some possibilities:

• Warn when you send and/or receive a zephyr encrypted with a weak key
• On startup, parse ~/.crypt-table, read the referenced keys, and warn on any weak keys
• Supply a script that parses ~/.crypt-table, reads the referenced keys, and warns on any weak keys