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

terminal.h

@@ -172,6 +172,7 @@ struct vt {
         uint8_t *data;
         size_t size;
         size_t idx;
+        bool bel; /* true if OSC string was terminated by BEL */
     } osc;
 
     /* Start coordinate for current OSC-8 URI */