internal/tlsconst: Add a package with TLS constants

There are a couple of places where it's handy to print TLS constants in
human-readable form.

To do so, we need functions that take TLS constants (usually in uint16 form)
and give us friendly strings.

Go's crypto/tls package does not provide us with this, so this patch
introduces a new module with that purpose.

internal/tlsconst/tlsconst.go

@@ -0,0 +1,32 @@
+// Package tlsconst contains TLS constants for human consumption.
+package tlsconst
+
+// Most of the constants get automatically generated from IANA's assignments.
+//go:generate ./generate-ciphers.py ciphers.go
+
+import "fmt"
+
+var versionName = map[uint16]string{
+	0x0300: "SSL-3.0",
+	0x0301: "TLS-1.0",
+	0x0302: "TLS-1.1",
+	0x0303: "TLS-1.2",
+}
+
+// VersionName returns a human-readable TLS version name.
+func VersionName(v uint16) string {
+	name, ok := versionName[v]
+	if !ok {
+		return fmt.Sprintf("TLS-%#04x", v)
+	}
+	return name
+}
+
+// CipherSuiteName returns a human-readable TLS cipher suite name.
+func CipherSuiteName(s uint16) string {
+	name, ok := cipherSuiteName[s]
+	if !ok {
+		return fmt.Sprintf("TLS_UNKNOWN_CIPHER_SUITE-%#04x", s)
+	}
+	return name
+}