Remove `force_reuse_port` from nyat-core

[uchouT]

Problem

force_reuse_port has two design issues that make it a poor fit for a library crate:

1. Blocking /proc scan in async context

force_reuse_port() synchronously iterates /proc/net/tcp and /proc/[pid]/fd/* to locate the process and fd occupying the target port. This runs inside the async mapper setup path.

2. Narrow privilege requirements

pidfd_getfd(2) requires the caller to have the same uid as the target process (or be root). In practice, this limits usage to services running
as root — a narrow scenario that does not justify the complexity in a general-purpose library.

Proposal

Move force_reuse_port out of nyat-core:

• Make LocalAddr::socket() pub, returning a bound socket to the caller
• nyat-core only sets SO_REUSEPORT on its own sockets (passive reuse)
• Active forced reuse becomes the caller's responsibility