smtpd: MAIL FROM/RCPT TO address validation is not our business

Next() is now out of the business of doing any address validation at
all. This is better done in callers. If the package provides address
validation as a service, it should be opt in and/or done with functions
that outside people can call themselves as they want.
Makefile: add something to test both the package and the command

'make test' is a lot more convenient than doing it by hand.
sinksmtp: fully transition to fully working rules

All of sinksmtp's various rejection and processing options now actually
create rules under the hood. This has enabled the deletion of a whole
pile of code and makes the core logic a lot simpler. A number of bugs
were shaken out in the process of making this switch, which is good.

This also brings us up to what should be a fully working and more or
less finalized rule engine and rule language.
sinksmtp/tlsnames.go: add a name for the 0 cipher

In case we ever wind up trying to print out the name for the 0x0000 zero
cipher, now I've given it one.