network: fix ProxySocket receive edge cases

Routing recv() through RecvFrom made several old RecvFrom problems
reachable from the normal read path:

- A partial TCP read kept zero bytes in front of the unread data.
- A blocking read busy-waited with yield() and reported ETIMEDOUT.
- MSG_PEEK hit the flags == 0 assert.
- A UDP datagram larger than the buffer was dropped with EMSGSIZE.

Wait on a condition variable signalled when a packet arrives, and report
an expired wait as EAGAIN like an SO_RCVTIMEO timeout. Keep the existing
5s cap for sockets without a receive timeout. Allow MSG_PEEK. Truncate
oversized datagrams as BSD recv() does, and keep the unread tail of a
TCP packet for the next read.

Assisted-by: Claude Opus 5.5 (investigation and implementation)

0 files changed, 0 insertions, 0 deletions