generate-accessors: add lifecycle, defaults, and annotation redesign

[martin-belanger]

Extend the accessor generator with three new capabilities.

1. Lifecycle (constructor + destructor): annotate a struct's opening brace with //!generate-lifecycle to generate foo_new() and foo_free(). foo_new() allocates a zeroed instance with calloc() and returns -EINVAL / -ENOMEM on error. foo_free() frees all owned char* and char** members then frees the struct. Passing NULL to foo_free() is safe: destructors that dereference members guard with if (!p) return; those with no members to dereference rely on free(NULL) being a no-op. //!lifecycle:none on a member excludes it from the destructor. const char* members are never freed (assumed externally owned).

2. Defaults (init function): annotate individual members with //!default:VALUE to generate foo_init_defaults(), which assigns each annotated field its compile-time default. Any valid C expression is accepted as the value. When combined with //!generate-lifecycle, foo_new() calls foo_init_defaults() after allocation. foo_init_defaults() is also useful standalone to re-initialise a struct without reallocating.

3. Annotation style: drop support for the /*!annotation*/ block-comment style. Annotations now use only the // line-comment style. The parser was redesigned accordingly: after //, each !keyword token is treated as a command, so multiple annotations may share one comment:

  struct foo { //!generate-accessors !generate-lifecycle

private.h and private-fabrics.h are updated throughout to use the new style.

The generated .c files now include <errno.h> for EINVAL/ENOMEM.

Apply these features to struct libnvmf_discovery_args, replacing the manually-written libnvmf_discovery_args_create() and libnvmf_discovery_args_free() with generated equivalents. The constructor is renamed _new (consistent with the generated naming convention).