in smtp submission, if a fromid is present in the mailfrom command, use it when queueing

it's the responsibility of the sender to use unique fromid's.
we do check if that's the case, and return an error if not.

also make it more clear that "unique smtp mail from addresses" map to the
"FromIDLoginAddresses" account config field.

based on feedback from cuu508 for #31, thanks!

testdata/smtpfromid/mox.conf

@@ -0,0 +1,9 @@
+DataDir: data
+User: 1000
+LogLevel: trace
+Hostname: mox.example
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster
+Listeners:
+	local: nil