dpll: Add dpll command

Add a new userspace tool for managing and monitoring DPLL devices via the
Linux kernel DPLL subsystem. The tool uses libmnl for netlink communication
and provides a complete interface for device and pin configuration.

The tool supports:

• Device management: enumerate devices, query capabilities (lock status,
  temperature, supported modes, clock quality levels), configure phase-offset
  monitoring and averaging

• Pin management: enumerate pins with hierarchical relationships, configure
  frequencies (including esync), phase adjustments, priorities, states, and
  directions

• Complex topologies: handle parent-device and parent-pin relationships,
  reference synchronization tracking, multi-attribute queries (frequency
  ranges, capabilities)

• ID resolution: query device/pin IDs by various attributes (module-name,
  clock-id, board-label, type)

• Monitoring: real-time display of device and pin state changes via netlink
  multicast notifications

• Output formats: both human-readable and JSON output (with pretty-print
  support)

The tool belongs in iproute2 as DPLL devices are tightly integrated with
network interfaces - modern NICs provide hardware clock synchronization
support. The DPLL subsystem uses the same netlink infrastructure as other
networking subsystems, and the tool follows established iproute2 patterns
for command structure, output formatting, and error handling.

Example usage:

1. dpll device show
2. dpll device id-get module-name ice
3. dpll device set id 0 phase-offset-monitor enable
4. dpll pin show
5. dpll pin set id 0 frequency 10000000
6. dpll pin set id 13 parent-device 0 state connected prio 10
7. dpll pin set id 0 reference-sync 1 state connected
8. dpll monitor
9. dpll -j -p device show