Track the last session for a client and add a flag to switch-client and

a key binding (L) to move a client back to its last session.

key-bindings.c

@@ -128,6 +128,7 @@ key_bindings_init(void)
 		{ '=', 			  0, &cmd_choose_buffer_entry },
 		{ '?', 			  0, &cmd_list_keys_entry },
 		{ 'D',			  0, &cmd_choose_client_entry },
+		{ 'L',			  0, &cmd_switch_client_entry },
 		{ '[', 			  0, &cmd_copy_mode_entry },
 		{ '\'',			  0, &cmd_command_prompt_entry },
 		{ '\002', /* C-b */	  0, &cmd_send_prefix_entry },