e2fsck: make the "fs is mounted; continue?" prompt more paranoid

A user received the "file system is mounted; do you really want to
continue" prompt, and then instead of typing "n" for no, forgot that
he hadn't declined to continuation question, and typed the up-arrow
key, which in his locale, the 'A' in "^[[A" was interpreted as "yes",
and he lost data.

This was clearly the user's fault, but to make e2fsck a bit safer
against user stupidity/carelessness, we will change the "fs is
mounted; continue?" prompt to default to no, and treat the escape
character (along with the return and space characters, currently) as a
request for the default answer.

Addresses-Debian-Bug: #619859
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/e2fsck/unix.c b/e2fsck/unix.c
index 624e11b..f4792a5 100644
--- a/e2fsck/unix.c
+++ b/e2fsck/unix.c
@@ -232,7 +232,7 @@
 	printf(_("\n\n\007\007\007\007WARNING!!!  "
 	       "The filesystem is mounted.   If you continue you ***WILL***\n"
 	       "cause ***SEVERE*** filesystem damage.\007\007\007\n\n"));
-	cont = ask_yn(_("Do you really want to continue"), -1);
+	cont = ask_yn(_("Do you really want to continue"), 0);
 	if (!cont) {
 		printf (_("check aborted.\n"));
 		exit (0);