userdb: correctly scan password when looking up user

shorter, cleaer error message on bad username

mailmux: use builtin sqlite datetime functions to set modtime Better to use the functionality provided by sqlite since we've decided to bring on the dependency. Might as well use it.

mailmux: store last modified time of users, aliases This is useful information that could be used for auditing people abusing the service, some basic non-invasive system metrics, and of course it's useful for users to see in the interface.

mailmux: deduplicate userdb table init

mailmux: limit registration request/response sizes Hopefully this will prevent any abuse from causing too many problems!

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Move server, non-cmd stuff out of cmd

mailmux: Actually try to create the database if it doesn't exist

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