osc: use BEL terminator in OSC replies to BEL-terminated OSC queries

This matches the documented (and observed) behavior in xterm:

> XTerm accepts either BEL or ST for terminating OSC sequences, and
> when returning information, uses the same terminator used in a query

-- https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h3-Operating-System-Commands

vt.c

@@ -579,6 +579,7 @@ action_osc_end(struct terminal *term, uint8_t c)
     if (!osc_ensure_size(term, term->vt.osc.idx + 1))
         return;
     term->vt.osc.data[term->vt.osc.idx] = '\0';
+    term->vt.osc.bel = c == '\a';
     osc_dispatch(term);
 }