libthread: add threadmaybackground

Programs that want to background themselves now need
to define threadmaybackground returning 1.
This avoids a confusing (to people and debuggers)
extra parent process for all the threaded programs
that will never want to background themselves.

src/cmd/ndb/dns.c

@@ -121,6 +121,12 @@ checkaddress(void)
 		fprint(2, "warning: announce mismatch %s %s\n", udpaddr, tcpaddr);
 }
 
+int
+threadmaybackground(void)
+{
+	return 1;
+}
+
 void
 threadmain(int argc, char *argv[])
 {