implement Dbus unregister/unlisten methods

implemented two related Dbus methods:
- unregister
- unlisten

improved error handling for Dbus method:
- link

fixed logic error in Manager.java (line 876) because daemon
could still be running after a particular account is unregistered.

broadened WebSocketUnavailableException (line 913) to catch all
IOExceptions caused by unregistering account while daemon is running.

specified error message handling in ProvisioningManager.java and
DbusSignalControlImpl.java for link subcommand.

helper methods:
- getPathConfig in Manager.java
- private removeUserData in DbusSignalImpl.java

man/signal-cli-dbus.5.adoc

@@ -114,6 +114,22 @@ sendNoteToSelfMessage(message<s>, attachments<as>) -> timestamp<x>::
 
 Exceptions: Failure, AttachmentInvalid
 
+unlisten() -> <>::
+unlisten(keepData<b>) -> <>::
+* keepData  : true or omitted = keep files in data directory; false = delete files
+
+Stops the current device from listening to DBus. In single-user mode, kills the daemon.
+
+Exception: Failure
+
+unregister() -> <>::
+unregister(keepData<b>) -> <>::
+* keepData  : true or omitted = keep files in data directory; false = delete files
+
+Unregisters the current device. In single-user mode, kills the daemon.
+
+Exception: Failure
+
 sendMessage(message<s>, attachments<as>, recipient<s>) -> timestamp<x>::
 sendMessage(message<s>, attachments<as>, recipients<as>) -> timestamp<x>::
 * message     : Text to send (can be UTF8)