tcp_acceptor::accept() should support returning the peer socket

[sgerbino]

Current API

corosio::tcp_socket peer(ioc);
auto [ec] = co_await acc.accept(peer);

The caller must pre-construct a tcp_socket and pass it to accept().

Proposed addition

Add an overload that returns the peer socket directly:

auto [ec, peer] = co_await acc.accept();

The existing accept(tcp_socket&) overload is retained for callers that want to manage socket lifetime themselves (e.g., pre-allocated or recycled sockets).

Motivation

The current interface requires the caller to construct a socket before accepting. This has a design issue: the caller can pass a socket associated with a different io_context than the acceptor's, creating a failure mode that doesn't exist at the POSIX level (where accept() simply returns a new file descriptor).

The acceptor already knows its io_context, and the underlying accept(2) returns a new fd — it doesn't write into a pre-existing one. A returning overload:

1. Eliminates a misuse vector — no way to pass a socket bound to the wrong io_context
2. Matches POSIX semantics — accept(2) returns a new fd
3. Simplifies common usage — one line instead of two

Retaining the existing overload gives callers who pre-allocate or recycle sockets a way to continue doing so.

Affected code

• include/boost/corosio/tcp_acceptor.hpp — add returning overload and corresponding awaitable
• Backend implementations (epoll, select, kqueue, iocp) — may need a new virtual or the returning overload can be built on top of the existing one