The spf library has gained support for macros, but to process them
properly, a new function needs to be called with the full sender
address, spf.CheckHostWithSender.
This patch updates chasquid's calls to the new API.
This patch adds some tooling and scripts to generate test coverage
information.
Unfortunately, this involves some hacks as Go does not have support for
generating coverage-enabled binaries, or merging coverage reports; but
overall it's not very intrusive.
This patch adds a package for evaluating SPF, as defined by RFC 7208
(https://tools.ietf.org/html/rfc7208).
It doesn't implement 100% of the RFC, but it coves enough to handle the
most common cases, and will fail open on the others.
