sockstat(1): use tree(3) rbtree instead of hash

[glebius]

Summary

• Use tree to lookup by socket kvaddr. The size of hash is too big for a small virtual machine and at the same time too little for a large production server. A tree would better fit here.
• Provide a second tree to lookup by pcb kvaddr. These removes full hash traversal when printing every unix(4) socket.

[glebius]

Preserve functionality of printing those pcbs, that don't have a socket.