Support Linux fork on Windows userland

[Weidong Cui (wdcui)]

This PR adds Linux-shim fork and vfork+exec support on Windows userland. The broker writes a forked child's memory image into a page-file-backed section that reserves the policy's maximum child image size and commits pages as they are written, and passes the child runner a read-only handle to it during setup. The runner maps short-lived views of the section to restore each region. To support this, ProcessLauncher::create_image now takes the image capacity, and --allow-process-duplication is now available on Windows. The Linux-on-Windows runner now decodes child startup data, either restoring a forked process from its image or starting an exec'd program in a new runner, so its program argument is now optional. New Windows loader tests run prebuilt static fork_parent and vfork_exec_parent/vfork_exec_child guests, and unit tests cover the image transport, including that the runner cannot write the image.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure trait_method_parameter_count_changed: pub trait method parameter count changed ---

Description:
A trait method now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/trait_method_parameter_count_changed.ron

Failed in:
  ProcessLauncher::create_image now takes 1 instead of 0 parameters, in file /home/runner/work/litebox/litebox/litebox_broker_host/src/lib.rs:905