chg: show debug message for each fd to be closed

It helps debugging. The number of file descriptors should be small in most
cases, so the console output wouldn't get bloated even with CHG_DEBUG=1.

contrib/chg/chg.c

@@ -289,6 +289,7 b' static void execcmdserver(const struct c'
 				continue;
 			}
 			if (fd_value > STDERR_FILENO) {
+				debugmsg("closing fd %ld", fd_value);
 				int res = close(fd_value);
 				if (res) {
 					debugmsg("tried to close fd %ld: %d "