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 adds configuration options for the MDA binary and command line
arguments, and changes the (soon to be renamed) procmail courier to make use
of them.
This patch adds a package implementing a simple user database, called userdb.
It has a human readable space-separated extensible format, and uses scrypt for
password storage (but supports plain as well, for debugging and testing).
chasquid is not using it yet, that will come in later patches.
