mda-lmtp: Add -to_puny, to punycode-encode addresses

Some LMTP servers (like dovecot) can't handle UTF8 addresses in the LMTP
commands. This can be problematic if we want to use them with UTF8
domains or usernames, which are well supported by chasquid.

To help workaround this issue, this patch adds a new -to_puny flag for
mda-lmtp, that makes it encode `from` and `recipient` in punycode.

That way, the server will get punycode-encoded (ASCII) strings in the
LTMP commands.

This can be particularly convenient when the recipients are ASCII
(because they're under the mail server control), but `from` may not be
(because it comes from the network).

cmd/mda-lmtp/test_puny_invalid.cmy

@@ -0,0 +1,9 @@
+mda = ./mda-lmtp --addr=.test-sock --addr_network=unix \
+        -to_puny -f fröm -d xn--t < test-email
+mda <- cannot puny-encode recipient: idna: invalid label "t"
+mda wait 2
+
+mda = ./mda-lmtp --addr=.test-sock --addr_network=unix \
+        -to_puny -f xn--f -d to < test-email
+mda <- cannot puny-encode from: idna: invalid label "f"
+mda wait 2