the panic log

Add an optional paniclog file, controlled by a new PANICLOG macro that can
be used to log all panic and impossible messages.  Helpful when people
forget to send, or didn't see, the message.

src/pline.c

@@ -248,6 +248,11 @@ void
 impossible VA_DECL(const char *, s)
 	VA_START(s);
 	VA_INIT(s, const char *);
+	{
+	    char pbuf[BUFSZ];
+	    Vsprintf(pbuf,s,VA_ARGS);
+	    paniclog("impossible", s);
+	}
 	vpline(s,VA_ARGS);
 	pline("Program in disorder - perhaps you'd better #quit.");
 	VA_END();