Fix non-printable keyname printing to use <octal> syntax.

The IsPrint() was grabbing values outside the range of a char.
Instead, restrict the range and fall back to the <octal> syntax, which
the muttrc actually accepts.

keymap.c

@@ -611,10 +611,10 @@ static const char *km_keyname (int c)
   }
   else if (c >= KEY_F0 && c < KEY_F(256)) /* this maximum is just a guess */
     sprintf (buf, "<F%d>", c - KEY_F0);
-  else if (IsPrint (c))
+  else if (c < 256 && c >= -128 && IsPrint (c))
     snprintf (buf, sizeof (buf), "%c", (unsigned char) c);
   else
-    snprintf (buf, sizeof (buf), "\\x%hx", (unsigned short) c);
+    snprintf (buf, sizeof (buf), "<%ho>", (unsigned short) c);
   return (buf);
 }