preload: Wrap fstatfs()

[martinpitt]

Revert commit 4b1ab11c244a0a488bb. Merely setting $SYSTEMD_DEVICE_VERIFY_SYSFS does not work for GI bindings. Instead, wrap fstatfs() and mangle the type to SYSFS_MAGIC if the path points into our mocked /sys. This satisfies udev.

Fixes https://github.com/systemd/systemd/issues/23499

[martinpitt]

@benzea WDYT?

[martinpitt]

I ran upower's integration tests against this, and they are happy now :tada:

[martinpitt]

This is a quickfix. Follow-ups:

• Factorize is_fd_in_testbed() or similar, to clean up macro
• Clean up strlen() + 5
  • ~Wrap statvfs()~ -- uninteresting
  • Wrap statfs()
  • Unit tests
  • Restore errno