client: Fix rogue execute protection

To protect against a rogue server reading your files, the client
honors C_EXECUTE only when it matches recent player input.

This has a somewhat troubled history, detailed in the previous commit.

The remaining major issue comes from commit 8b7d0b9 (v4.3.11): any
suffix of a recent line of input is accepted as C_EXECUTE text.
Before, only text that looked like an argument of an execute command
or a redirection was accepted.

Fix by again requiring the text to be preceded by something that looks
like an execute command.  But do it more carefully: don't break
execute with a prompted for argument, and prevent abuse of
redirections for execute.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/client/ringbuf.c

@@ -157,16 +157,17 @@ ring_discard(struct ring *r, int n)
 
 /*
  * Search the ring buffer for zero-terminated string S.
- * If found, return a non-negative value referring to the beginning of
- * S in the buffer when passed to ring_peek().  Else return -1.
+ * Start at the @(n+1)-th byte to be gotten.
+ * If found, return the number of bytes in the buffer before S.
+ * Else return -1.
  */
 int
-ring_search(struct ring *r, char *s)
+ring_search(struct ring *r, char *s, int n)
 {
     size_t len = strlen(s);
     size_t i, j;
 
-    for (i = r->cons; i + len <= r->prod; i++) {
+    for (i = r->cons + n; i + len <= r->prod; i++) {
 	for (j = 0; s[j] && s[j] == (char)r->buf[(i + j) % RING_SIZE]; j++)
 	    ;
 	if (!s[j])