(feat): Implement functional Layer 1 keepalive support for IOU/IOL nodes

[cristian-ciobanu]

Summary

Implement functional Layer 1 keepalive support for IOU/IOL nodes so interface status accurately reflects whether a link is connected.

Purpose

Without L1 keepalives, IOU/IOL interfaces may remain up when disconnected or report the wrong interface as connected. This affects both Ethernet and serial interfaces on IOL-L2 and IOL-L3 nodes.

Changes

• Added a UNIX datagram endpoint for exchanging L1 keepalive messages with IOU/IOL.
• Periodically sends keepalives for interfaces with an attached NIO.
• Responds to valid keepalives only when the corresponding interface is connected.
• Supports both Ethernet and serial adapters using IOU's global adapter numbering.
• Uses the correct IOU interface encoding:
  • Zero-based port/unit in the high nibble.
  • Zero-based adapter/bay in the low nibble.
• Starts the responder only when the IOU image supports and enables the -l option.
• Stops the responder and removes its socket when the node stops, terminates, or fails to start.
• Validates socket ownership before creating the endpoint.
• Updated API descriptions for the L1 keepalive option.
• Added regression tests for:
  • Ethernet and serial interface numbering.
  • Mixed Ethernet/serial topologies.
  • Connected and disconnected interfaces.
  • Malformed or unrelated keepalive messages.
  • Unsupported L1 capability.
  • Startup failure and repeated cleanup.

Examples

Interface	Keepalive interface ID
Ethernet0/0	0x00
Ethernet0/2	0x20
Ethernet1/1	0x11
Ethernet2/2	0x22
Serial4/0	0x04

Validation

• IOU VM test suite: 49 passed
• IOU API assertions: 31 passed

Tested using latest available IOL L2 and L3 images 17.18.02 version but I assume it should work with other versions.

Fixes #14