Add a key to delete to end of line at the prompt (^K in emacs mode, C/D in vi).

From Kalle Olavi Niemitalo.

status.c

@@ -823,6 +823,12 @@ status_prompt_key(struct client *c, int key)
 			c->flags |= CLIENT_STATUS;
 		}
 		break;
+	case MODEKEYCMD_DELETETOENDOFLINE:
+		if (c->prompt_index < size) {
+			c->prompt_buffer[c->prompt_index] = '\0';
+			c->flags |= CLIENT_STATUS;
+		}
+		break;
 	case MODEKEYCMD_UP:
 		if (server_locked)
 			break;