libbb: use _exit, not exit, in bb_daemonize_or_rexec()

By the time we reach exit in parent, child already exited or execed.
We should not re-run libc cleanup code.

While at it, introduce bb_daemon_helper() and add a few comments.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

printutils/lpd.c

@@ -198,9 +198,8 @@ int lpd_main(int argc UNUSED_PARAM, char *argv[])
 				q = p; // next line
 			}
 			// helper should not talk over network.
-			// this call reopens stdio fds to "/dev/null"
-			// (no daemonization is done)
-			bb_daemonize_or_rexec(DAEMON_DEVNULL_STDIO | DAEMON_ONLY_SANITIZE, NULL);
+			// this call reopens stdio fds to "/dev/null".
+			bb_daemon_helper(DAEMON_DEVNULL_STDIO);
 			BB_EXECVP_or_die(argv);
 		}