MINOR: cli: implements 'quit' in the CLI proxy

Implements the 'quit' command. Works the same way as the CLI command.
diff --git a/src/cli.c b/src/cli.c
index 3171c0a..c6d2265 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1787,6 +1787,11 @@
 	} else if (!strcmp("prompt", args[0])) {
 		s->pcli_prompt ^= 1;
 		return argl; /* return the number of elements in the array */
+
+	} else if (!strcmp("quit", args[0])) {
+		channel_shutr_now(&s->req);
+		channel_shutw_now(&s->res);
+		return argl; /* return the number of elements in the array */
 	}
 
 	return 0;