add User-Agent string as per RFC 8555 and RFC 7231

lib/browser/client-user-agent.js

@@ -0,0 +1,6 @@
+'use strict';
+
+var UserAgent = module.exports;
+UserAgent.get = function () {
+  return false;
+};

lib/node/client-user-agent.js

@@ -0,0 +1,34 @@
+'use strict';
+
+var os = require('os');
+
+var UserAgent = module.exports;
+UserAgent.get = function(me) {
+	// ACME clients MUST have an RFC7231-compliant User-Agent
+	// ex: Greenlock/v3 ACME.js/v3 node/v12.0.0 darwin/17.7.0 Darwin/x64
+	//
+	// See https://tools.ietf.org/html/rfc8555#section-6.1
+	// And https://tools.ietf.org/html/rfc7231#section-5.5.3
+	// And https://community.letsencrypt.org/t/user-agent-flag-explained/3843/2
+
+	var ua =
+		'ACME.js/v3 ' +
+		process.release.name +
+		'/' +
+		process.version +
+		' ' +
+		os.platform() +
+		'/' +
+		os.release() +
+		' ' +
+		os.type() +
+		'/' +
+		process.arch;
+
+	var pkg = me.packageAgent;
+	if (pkg) {
+		ua = pkg + ' ' + ua;
+	}
+
+	return ua;
+};