server: free strduped port numbers

Although the leak is negligible, the clean heap on exit will ease
valgrind testing.

Change-Id: I3a7a9c8e8dc7557aa51d0b9caa244537e5e7007d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4410
Tested-by: jenkins

src/server/server.h

@@ -82,6 +82,7 @@ int add_service(char *name, const char *port,
 int server_preinit(void);
 int server_init(struct command_context *cmd_ctx);
 int server_quit(void);
+void server_free(void);
 void exit_on_signal(int);
 
 int server_loop(struct command_context *command_context);