make xfunctions optionally longjump instead of exit.

use it for making NOFORK more practical.
touch: make it a NOFORK applet

libbb/xfuncs.c

@@ -476,7 +476,7 @@ void xprint_and_close_file(FILE *file)
 	fflush(stdout);
 	// copyfd outputs error messages for us.
 	if (bb_copyfd_eof(fileno(file), 1) == -1)
-		exit(xfunc_error_retval);
+		sleep_and_die();
 
 	fclose(file);
 }