#13 Add milter version 3 and 4 protocol support

[Todd Vierling]

This is a protocol version bump used to indicate
SMFIC_DATA and SMFIC_UNKNOWN capabilities. The milter
writer may need to indicate this directly, or it may be
possible to autodetect presence of "unknown" and "data"
callback handlers to choose version 3 and 4,
respectively, as a requirement.

On the callback side of things, so long as the commands
are consumed transparently, it should be possible to
support any SMFI_VERSION 2 through 4 automatically.