chasquid: Introduce support for SMTPUTF8

This patch adds initial support for SMTPUTF8, which for now consists of just
advertising it.

We support most of it, but sending emails over SMTP requires further work, as
the SMTP courier does not support this yet (it's not in Go's standard
library). That will come in subsequent patches, along with IDNA handling.

https://tools.ietf.org/html/rfc6531.html

chasquid.go

@@ -534,6 +534,7 @@ func (c *Conn) EHLO(params string) (code int, msg string) {
 	fmt.Fprintf(buf, c.hostname+" - Your hour of destiny has come.\n")
 	fmt.Fprintf(buf, "8BITMIME\n")
 	fmt.Fprintf(buf, "PIPELINING\n")
+	fmt.Fprintf(buf, "SMTPUTF8\n")
 	fmt.Fprintf(buf, "SIZE %d\n", c.maxDataSize)
 	if c.onTLS {
 		fmt.Fprintf(buf, "AUTH PLAIN\n")