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/generate-ciphers.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+#
+# This hacky script generates a go file with a map of version -> name for the
+# entries in the TLS Cipher Suite Registry.
+
+import csv
+import urllib.request
+import sys
+
+# Where to get the TLS parameters from.
+# See http://www.iana.org/assignments/tls-parameters/tls-parameters.xml.
+URL = "https://www.iana.org/assignments/tls-parameters/tls-parameters-4.csv"
+
+
+def getCiphers():
+	req = urllib.request.urlopen(URL)
+	data = req.read().decode('utf-8')
+
+	ciphers = []
+	reader = csv.DictReader(data.splitlines())
+	for row in reader:
+		desc = row["Description"]
+		rawval = row["Value"]
+
+		# Just plain TLS values for now, to keep it simple.
+		if "-" in rawval or not desc.startswith("TLS"):
+			continue
+
+		rv1, rv2 = rawval.split(",")
+		rv1, rv2 = int(rv1, 16), int(rv2, 16)
+
+		val = "0x%02x%02x" % (rv1, rv2)
+		ciphers.append((val, desc))
+
+	return ciphers
+
+
+ciphers = getCiphers()
+
+out = open(sys.argv[1], 'w')
+out.write("""\
+package tlsconst
+
+// AUTOGENERATED - DO NOT EDIT
+//
+// This file was autogenerated by generate-ciphers.py.
+
+var cipherSuiteName = map[uint16]string{
+""")
+
+for ver, desc in ciphers:
+	out.write('\t%s: "%s",\n' % (ver, desc))
+
+out.write('}\n')