Reuse fd in etc file generation and run unlink in thread

PR: https://github.com/truenas/middleware/pull/8466

This contains a few improvements to permissions management in
etc plugin:

• write_if_changed is updated to allow passing an fd rather than
  only a path name

• permissions check and file contents check are now run from same
  thread.

• permissions are set on file before writing.

• set default permissions on config files to 0644 unless dev
  specifies something different.