This patch adds a new test which makes chasquid send and receive email to/from
Exim.
To make it work we need to add two testing flags to the SMTP courier, which is
not ideal but doesn't muddle the code much.
The test is not very portable, and assumes an exim binary is available, but
does not have to be installed in the system. It includes a helper script to
fetch one from the Debian archives.
This patch does various minor style and simplification cleanups, fixing things
detected by tools such as go vet, gofmt -s, and golint.
There are no functional changes, this change is purely cosmetic, but will
enable us to run those tools more regularly now that their output is clean.
This patch introduces the couriers, which the queue uses to deliver mail.
We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.
There are still a few things pending, but they all have their basic
functionality working and tested.
