zebra: Add a subqueue2str function to give more useful data in debugs

zebra: Add a subqueue2str function to give more useful data in debugs

New output example:

2022-07-03 09:40:29.310 [DEBG] zebra: [JF0K0-DVHWH] rib_meta_queue_add: (0:254):4.5.6.8/32: queued rn 0x55937f586ee0 into sub-queue Kernel Routes
2022-07-03 09:40:29.321 [DEBG] zebra: [HH6N2-PDCJS] default(0:254):4.5.6.8/32 rn 0x55937f586ee0 dequeued from sub-queue Kernel Routes

Let's make it a bit more human readable.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>