Add missing Close calls

The SMTP courier was not properly closing the connection, and chasquid's
closing of incoming connections was not ideal (it was closing the
underlying one, not necessarily the active one, like in the case of a jump
to TLS).

This patch fixes both by adding the missing calls to Close.

chasquid.go

@@ -445,8 +445,12 @@ type Conn struct {
 	commandTimeout time.Duration
 }
 
+func (c *Conn) Close() {
+	c.netconn.Close()
+}
+
 func (c *Conn) Handle() {
-	defer c.netconn.Close()
+	defer c.Close()
 
 	c.tr = trace.New("SMTP.Conn", c.netconn.RemoteAddr().String())
 	defer c.tr.Finish()