Allow Plugin.start() to fail gracefully; removed unnecessary locking.

src/net/sf/briar/api/plugins/Plugin.java

src/net/sf/briar/plugins/bluetooth/BluetoothPlugin.java

src/net/sf/briar/plugins/droidtooth/DroidtoothPlugin.java

src/net/sf/briar/plugins/file/RemovableDrivePlugin.java

src/net/sf/briar/plugins/modem/ModemPlugin.java

src/net/sf/briar/plugins/tcp/LanTcpPlugin.java

src/net/sf/briar/plugins/tcp/TcpPlugin.java

src/net/sf/briar/plugins/tor/TorPlugin.java