revokefs-fuse: Add --with-exit-fd arg to monitor parent process's exit

This is necessary so as to not leave the revokefs backend around
when the system-helper exits abruptly (e.g. OOM killer). It would
be a vulnerability if revokefs backend continues to live even after
the system-helper is killed as it might lead to write access to the
underlying directory.

Closes: #2657
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f