mda-lmtp: Handle "<>" addresses

Either the recipient or from addresses can be "<>" to indicate the null
address. mda-lmtp does not handle that well, as it sends "<<>>" which is
invalid.

This patch fixes that by special-casing them, which is unfortunate but
reasonably common.

cmd/mda-lmtp/test_tcp_null.cmy

@@ -0,0 +1,32 @@
+
+nc tcp_listen localhost:14932
+
+mda |= ./mda-lmtp --addr=localhost:14932 -f "<>" -d "<>" < .data
+
+nc -> 220 Hola desde expect
+
+nc <~ LHLO .*
+nc -> 250-Bienvenido!
+nc -> 250 Contame...
+
+nc <- MAIL FROM:<>
+nc -> 250 Aja
+
+nc <- RCPT TO:<>
+nc -> 250 Aja
+
+nc <- DATA
+nc -> 354 Dale
+
+nc <- Subject: test
+nc <-
+nc <- This is a test.
+nc <- .
+
+nc -> 250 Recibido
+
+nc <- QUIT
+nc -> 221 Chauchas
+
+mda wait 0
+