socket: enforce socketpair_tcp()'s anti-hijack guarantee

socket: enforce socketpair_tcp()'s anti-hijack guarantee

socketpair_tcp() fakes a connected socket pair via a loopback TCP
self-connect (socket -> bind 127.0.0.1:0 -> listen -> connect ->
accept), used by sock_exec() for RSYNC_CONNECT_PROG. Its comment has
long promised that "nobody else can attach to the socket, or if they
do that this function fails", but nothing actually verified it: the
code accept()ed whatever connection arrived first without checking it
was the one our own connect() made.

Between listen() and accept() the ephemeral loopback port is
connectable by any local user. With backlog 1 a same-host attacker who
races a connection in before our connect() lands could have their
socket returned by accept(), handing them one end of the rsync
protocol stream. The exposure is small (loopback only, random
ephemeral port, sub-millisecond window, local users only), but the
promised guarantee was simply not enforced.

Enforce it: after the connection is established, require that the peer
address of the accepted end (fd[0]) equals the local address of our
connecting end (fd[1]), and that both are 127.0.0.1. A hijacked
connection has a different source port and is rejected (errno EPERM,
fail closed). The legitimate self-connect always matches, so there is
no behaviour change for the normal path.

Verified: rebuilds clean with -Wall -W; the full testsuite still
passes in both transports (pipe `make check` 57/3, `runtests.py
--use-tcp` 59/1) -- the pipe transport exercises this code path on
every daemon test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>