initial Resolute Raccoon stemcell support

[mkocher]

PoC changes to make this release work on a Resolute Raccoon stemcell. Blobs will need to be added to the blobstore by the maintainers.

AI Slop Summary of Changes:

Fix garden-runc-release compilation on Resolute Raccoon (Ubuntu 26.04)

Summary

Resolute Raccoon ships GCC 14/15, CMake 4.2, defaults to C23, and removes
pkg-config in favor of pkgconf. These changes break compilation of
several packages in garden-runc-release. This PR fixes all of them while
maintaining backward compatibility with Jammy.

Changes

• Replace pkg-config with pkgconf — delete the old pkg-config
  package, add a new pkgconf package with a pkg-config symlink, update
  all 6 dependent packages (iptables, lzo, runc, util-linux,
  xfs-progs, zlib)
• Bump xfsprogs from 4.20.0 to 6.19.0 for GCC 14 compatibility
• Add inih and userspace-rcu packages — new required dependencies
  for xfsprogs 6.x
• Build inih and userspace-rcu as static-only to avoid runtime
  shared-library resolution failures when grootfs calls mkfs.xfs
• Wrap mkfs.xfs to disable nrext64, exchange, and parent
  features that require kernels newer than our targets (Jammy 5.15,
  bosh-lite 6.8)
• Fix tini build with -DCMAKE_POLICY_VERSION_MINIMUM=3.5 for
  CMake 4.2
• Fix nstar.c — replace <linux/fcntl.h> with <fcntl.h> plus
  AT_EMPTY_PATH fallback for cross-version compatibility (in guardian
  submodule)
• Fix shell strictness in xfs-progs packaging script (uninitialized
  variable references under set -u)

Note: The nstar.c fix is in the src/guardian submodule. The
submodule reference must be updated to include this commit.