command: use the bool datatype instead of int

Instead of returning 0 for success and -1 for failure, return true or
false.  This seems more natural.

src/command.h

@@ -23,6 +23,8 @@
 #include "sllist.h"
 #include "ack.h"
 
+#include <stdbool.h>
+
 enum command_return {
 	COMMAND_RETURN_ERROR = -1,
 	COMMAND_RETURN_OK = 0,
@@ -38,7 +40,7 @@ void command_finish(void);
 
 enum command_return
 command_process_list(struct client *client,
-		     int list_ok, struct strnode *list);
+		     bool list_ok, struct strnode *list);
 
 enum command_return
 command_process(struct client *client, char *commandString);