send: Always MIMEify and pass already MIMIfied drafts through

Even if technically not necessary, MIMEify the draft. This simplifies
the code at hardly any cost.

To still be able to create arbitrary complex MIME structures, make
send(1) pass already MIMEified drafts through, without automatically
invoking mhbuild(1) on them a second time (which would fail). This
way, you can manually invoke mhbuild(1) and afterward call send(1)
... and everything will work just as expected.

Also documented this in the man page, and additionally added a
paragraph about the signing and encrypting feature of send(1), which
wasn't documented yet.