smtpsrv: Strict CRLF enforcement in DATA contents

The RFCs are very clear that in DATA contents:

> CR and LF MUST only occur together as CRLF; they MUST NOT appear
> independently in the body.

https://www.rfc-editor.org/rfc/rfc5322#section-2.3
https://www.rfc-editor.org/rfc/rfc5321#section-2.3.8

Allowing "independent" CR and LF can cause a number of problems.

In particular, there is a new "SMTP smuggling attack" published recently
that involves the server incorrectly parsing the end of DATA marker
`\r\n.\r\n`, which an attacker can exploit to impersonate a server when
email is transmitted server-to-server.

https://www.postfix.org/smtp-smuggling.html
https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/

Currently, chasquid is vulnerable to this attack, because Go's standard
libraries net/textproto and net/mail do not enforce CRLF strictly.

This patch fixes the problem by introducing a new "dot reader" function
that strictly enforces CRLF when reading dot-terminated data, used in
the DATA input processing.

When an invalid newline terminator is found, the connection is aborted
immediately because we cannot safely recover from that state.

We still keep the internal representation as LF-terminated for
convenience and simplicity.

However, the MDA courier is changed to pass CRLF-terminated lines, since
that is an external program which could be strict when receiving email
messages.

See https://github.com/albertito/chasquid/issues/47 for more details and
discussion.

internal/normalize/normalize_test.go

@@ -129,6 +129,22 @@ func TestDomainToUnicode(t *testing.T) {
 	}
 }
 
+func TestToCRLF(t *testing.T) {
+	cases := []struct {
+		in, out string
+	}{
+		{"", ""},
+		{"a\nb", "a\r\nb"},
+		{"a\r\nb", "a\r\nb"},
+	}
+	for _, c := range cases {
+		got := string(ToCRLF([]byte(c.in)))
+		if got != c.out {
+			t.Errorf("ToCRLF(%q) = %q, expected %q", c.in, got, c.out)
+		}
+	}
+}
+
 func FuzzUser(f *testing.F) {
 	f.Fuzz(func(t *testing.T, user string) {
 		User(user)