Add unregister command

Fixes #57

man/signal-cli.1.txt

@@ -66,6 +66,13 @@ Verify the number using the code received via SMS or voice.
 VERIFICATIONCODE::
 	The verification code.
 
+unregister
+~~~~~~~~
+Disable push support for this device, i.e. this device won't receive any more messages.
+If this is the master device, other users can't send messages to this number anymore.
+Use "updateAccount" to undo this.
+To remove a linked device, use "removeDevice" from the master device.
+
 link
 ~~~~
 Link to an existing device, instead of registering a new number.  This shows a

src/main/java/org/asamk/signal/Main.java

@@ -138,6 +138,22 @@ public class Main {
                         return 3;
                     }
                     break;
+                case "unregister":
+                    if (dBusConn != null) {
+                        System.err.println("unregister is not yet implemented via dbus");
+                        return 1;
+                    }
+                    if (!m.isRegistered()) {
+                        System.err.println("User is not registered.");
+                        return 1;
+                    }
+                    try {
+                        m.unregister();
+                    } catch (IOException e) {
+                        System.err.println("Unregister error: " + e.getMessage());
+                        return 3;
+                    }
+                    break;
                 case "verify":
                     if (dBusConn != null) {
                         System.err.println("verify is not yet implemented via dbus");
@@ -679,6 +695,9 @@ public class Main {
                 .help("The verification should be done over voice, not sms.")
                 .action(Arguments.storeTrue());
 
+        Subparser parserUnregister = subparsers.addParser("unregister");
+        parserUnregister.help("Unregister the current device from the signal server.");
+
         Subparser parserVerify = subparsers.addParser("verify");
         parserVerify.addArgument("verificationCode")
                 .help("The verification code you received via sms or voice call.");

src/main/java/org/asamk/signal/Manager.java

@@ -356,6 +356,13 @@ class Manager implements Signal {
         save();
     }
 
+    public void unregister() throws IOException {
+        // When setting an empty GCM id, the Signal-Server also sets the fetchesMessages property to false.
+        // If this is the master device, other users can't send messages to this number anymore.
+        // If this is a linked device, other users can still send messages, but this device doesn't receive them anymore.
+        accountManager.setGcmId(Optional.<String>absent());
+    }
+
     public URI getDeviceLinkUri() throws TimeoutException, IOException {
         password = Util.getSecret(18);