go-milter: build it patched not to keep every header

The security documentation's last open item of panel-dkim-signing (the
owner's decision, 2026-10-16): go-milter v0.4.1 added every header of a
session to a map of its own, emptied only on ABORT, and handed it to
Headers - which no milter of the panel reads. Postfix bounds one header,
not their number, so a message of nothing but headers up to the message
size limit was held whole by the library in the sender check, the signer
and the journal milter on 465/587 and in the inbound journal milter on port
25, where strangers send; the signer's 1 MiB refusal came after it was all
in.

third_party/go-milter is v0.4.1 from the module cache (BSD 2-Clause,
LICENSE kept) with session.go changed: CodeHeader hands the header to
Header and keeps nothing, CodeEOH hands Headers an empty map. PATCHES.md
says what and why, and how to take a new upstream version. go.mod replaces
the module with it; the Dockerfile copies third_party before "go mod
download". TestTheLibraryKeepsNoHeaders holds it through the library's own
client over two messages of one connection. Comments, security.md (the
accepted risk becomes a note), development.md's library table, the plan and
CHANGELOG follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

third_party/go-milter/cstrings.go

@@ -0,0 +1,33 @@
+package milter
+
+import (
+	"bytes"
+	"strings"
+)
+
+// NULL terminator
+const null = "\x00"
+
+// DecodeCStrings splits a C style strings into a Go slice
+func decodeCStrings(data []byte) []string {
+	if len(data) == 0 {
+		return nil
+	}
+	return strings.Split(strings.Trim(string(data), null), null)
+}
+
+// ReadCString reads and returns a C style string from []byte
+func readCString(data []byte) string {
+	pos := bytes.IndexByte(data, 0)
+	if pos == -1 {
+		return string(data)
+	}
+	return string(data[0:pos])
+}
+
+// appendCString appends a C style string to the buffer and returns it (like append does).
+func appendCString(dest []byte, s string) []byte {
+	dest = append(dest, []byte(s)...)
+	dest = append(dest, 0x00)
+	return dest
+}