libnvme: commit pre-generated accessor files and simplify build

[martin-belanger]

The accessor functions (getters/setters for nvme_path, nvme_ns, nvme_ctrl, nvme_subsystem, nvme_host, nvme_fabric_options) were previously generated at build time by a custom_target in Meson. They are now committed to the source tree and treated as maintainer-generated source files, similar to how the Linux kernel and systemd handle infrequently-changing generated code.

Changes:

• Commit accessors.h, accessors.c, accessors.ld to the source tree
• Remove the custom_target that ran the generator on every build; generate-accessors is no longer compiled during a normal build
• Add accessors.c to the sources list and accessors.h to install_headers() alongside their sibling files in libnvme/src/meson.build
• Remove accessors_dep (was only needed to forward the custom_target outputs; plain files don't require this indirection)
• Enhance generate-accessors.c to emit mechanical Doxygen stubs in accessors.h, covering all three member kinds: value types, dynamic char * (with NULL/copy semantics), and fixed char arrays
• Add update-accessors.sh: atomically regenerates accessors.h and accessors.c when content changes, and reports symbol additions and removals against accessors.ld without overwriting it
• Add a Meson run_target and Makefile target 'update-accessors' for on-demand regeneration: make update-accessors
• Keep accessors.ld manually maintained; its version section labels (e.g. LIBNVME_ACCESSORS_3_0) must be assigned by the maintainer to preserve ABI compatibility
• Add .github/workflows/check-accessors.yml to fail CI when the committed accessor files drift from the generator output
• Update README.md with a developer guide covering regeneration, accessors.ld maintenance, and the ABI versioning convention
• Fix three stale test reference files that were missing dhchap_ctrl_key from their expected JSON output following an earlier json.c update
• Made several changes to generate-accessors.c to make sure generated code complies to checkpatch rules

[igaw]

Thanks a lot. I think this is a good middle ground. And getting rid of the hand written stuff is good :)