sinksmtp: add option for how far a non-do-nothing client needs to get

-minphase says that a client must successfully get to whatever in order
to not be considered a do-nothing client. The old state (equivalent to
'-minphase helo') is the default. Does nothing without -dncount and so
on.
sinksmtp: revise the manual page comment at the start of sinksmtp.go

In the process I've somehow made godoc not recognize it as documentation.
I'm going to have to get people who know more about Godoc than I do to
tell me what I'm doing wrong, because it sure looks like eg go vet's
documentation to me.