ed: Don't undo commands in sigint

If newcmd is 0 then error() undo all the modifications
that happened since the last command, but this is  not
what POSIX mandates:

SIGINT	The ed utility shall interrupt its current activity, write the
	string "?\n" to standard output, and return to command mode
	(see the EXTENDED DESCRIPTION section).

ed.c

@@ -740,6 +740,7 @@ chksignals(void)
 
 	if (intr) {
 		intr = 0;
+		newcmd = 1;
 		clearerr(stdin);
 		error("Interrupt");
 	}