mu-server: implement temp-file optimization

It can be faster to feed big mu -> mu4e data, such as contacts are
message headers through a temp-file instead directly though stdout;
implement this, and add the server parameter --allow-temp-file.

Implement this the "contacts" and "find" commands.

mu/mu-options.hh

@@ -222,6 +222,7 @@ struct Options {
 	struct Server {
 		bool		commands;		/**< dump docs for commands */
 		std::string	eval;			/**< command to evaluate */
+		bool            allow_temp_file;	/**< temp-file optimization allowed? */
 	} server;
 
 	/*