executor: improve opendir(3) error handling

While investigating an OpenBSD reproducer[1][2] I discovered the
following:

* All threads are stuck on the last `sleep(1000000)` syscall in main(),
  hence no output for the test machine.

* Each executor process created in loop() performs one iteration but
  exits abnormally during the call to remove_dir().

* Calling remove_dir() will eventually invoke itself recursively since
  one of the executed syscall is `mkdir("./file0", 0)` meaning that it
  will try to remove the directory created by execute_one(). However,
  `opendir(3)` fails with `EACCES` due to the permissions passed to
  `mkdir(2)` is zero.

Instead of exiting, trying to remove the problematic directory in a best
effort manner makes the reproducer continue executing the generated
syscalls. This work around might be considered to narrow. Another option
would be to replace the `sleep(1000000)` with `waitpid(-1, NULL, 0)`
until ECHILD is hit.

[1] https://syzkaller.appspot.com/bug?id=6f7ce2a0536580a94f65f44e478732ec505e88af
[2] https://syzkaller.appspot.com/text?tag=ReproC&x=10fd1a71900000

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