Document emgr_check_ifixes selection logic and add machine-readable output with supersedence and reboot metadata

The emgr_check_ifixes command is a natural starting point for automating AIX security interim fix (iFix) management, but its documentation describes only how to invoke it — not how it selects the fixes it reports, nor the format of its output. In practice the command applies several undocumented selection steps and omits metadata essential for automation.

Observed gaps between documented and actual behavior:

• Scope - The documentation states it checks the availability of security iFixes for the current OS level. In practice it reports only fixes for products already installed, at a version within the advisory's vulnerable range — i.e. what is applicable to this system now, not everything available for the OS level. These are different sets, and the distinction matters for fleet planning and image-build workflows.
• Signature validation - Advisories that fail signature validation are skipped with no message to the user.
• Fix age - An age-based heuristic can omit older fixes, potentially excluding legitimately applicable content.
• Output - Plain tab-separated text, derived from an undocumented upstream data format, with no supported machine-readable option.
• Metadata - The output carries neither supersedence relationships nor reboot requirements — both essential for safe, unattended patching.

Requested enhancements:

1. Document the command's fix-selection logic.
2. Distinguish "available for the OS level" from "applicable to this system", and offer both.
3. Provide a supported, documented machine-readable output (e.g. JSON).
4. Include supersedence and reboot metadata in the output.
5. Surface, rather than silently skip, advisories that fail signature validation.
6. Document and make optional the fix-age heuristic.

Business value - Security patching of a large AIX estate must be automated to be timely, consistent and auditable. These gaps require customers to build and maintain custom tooling — parsing an undocumented format and deriving reboot/supersedence information independently. Addressing them would let customers rely on supported IBM capabilities, reduce the risk of missing security-relevant fixes, and lower the maintenance burden of enterprise AIX patching.