Recognise and discard the APC terminal escape sequence.

I encountered an instance of this sequence in the log files from a
clang CI build. The payload text inside the wrapper was
"bk;t=1697630539879"; I don't know what the "bk" stood for, but the
second half appears to be a timestamp in milliseconds since the Unix
epoch.

I don't think there's anything we can (or should) actually _do_ with
this sequence, but I think it's useful to at least recognise it, so
that it can be conveniently discarded.

terminal/terminal.h

@@ -182,6 +182,7 @@ struct terminal_tag {
 #define ANSI_QUE(x)     ANSI(x,1)
 
 #define OSC_STR_MAX 2048
+    bool osc_is_apc;
     int osc_strlen;
     char osc_string[OSC_STR_MAX + 1];
     bool osc_w;