test(net): cover nonblocking socket read and readv semantics

[fslongjin]

Summary

Add eight dunitest regression cases for FIONBIO-controlled read and readv on TCP, UDP, Unix stream and Unix datagram sockets.

The nonblocking state synchronization fix merged in #2317 also resolves the nginx HTTPS handshake-success/HTTP-timeout symptom. This PR adds missing file-style read coverage; it does not introduce another kernel change or an nginx workaround.

Coverage and design

• Enable nonblocking mode and verify shared flags through dup.
• Require EAGAIN on an empty receive queue.
• Read an available byte, then require EAGAIN after draining.
• Clear FIONBIO through the alias and verify that a read waits for delayed data.
• Reuse existing socket ownership and bounded child-process helpers, with no new dependencies.
• Use single-byte writes rather than assuming a stream read fills its buffer.

Validation

• Linux: 19/19 socket_nonblocking cases pass.
• Controlled guest comparison restoring the previous nonblocking implementation: all eight new cases detect incorrect blocking and are bounded by the existing watchdog.
• Current DragonOS: 19 socket_nonblocking, 13 socket_readv_semantics, 3 tcp_listen_poll_semantics and 40 tcp_relisten cases pass (75 total).
• Real nginx on DragonOS: guest curl succeeds with TLS 1.2 and TLS 1.3, including certificate verification.
• Host-to-guest HTTPS: 20 connections per TLS version, five requests per connection, concurrency four, 100ms gaps; all 200 requests pass with correct body and verified keepalive reuse.
• The controlled old-behavior comparison completes both TLS handshakes but times out awaiting HTTP responses; a syscall snapshot captures a worker inside read. The comparison isolates the nonblocking change group, not each individual modification.
• make kernel, FMT_CHECK=1 make fmt, and git diff --check pass.
• Independent design and implementation review completed with no blocking findings.

The HTTPS checks cover IPv4 HTTP/1.1 with a short static response; they are not a claim of complete TLS or nginx compatibility.

[fslongjin]

@codex review