feat(net, veb): add OpenSSL-based HTTPS server support

[guweigang]

Summary

This PR implements OpenSSL-based HTTPS server capabilities in net.openssl and integrates it into the veb web framework. Previously, veb only supported running HTTPS servers using net.mbedtls. When compiling with -d use_openssl, veb HTTPS servers were unavailable and would return a compile/runtime error.

Details of Changes

1. net.openssl:

   • Added missing OpenSSL C bindings: C.TLS_server_method, C.SSL_CTX_check_private_key, C.SSL_accept, and C.ERR_print_errors_fp.
   • Implemented SSLListener along with new_ssl_listener(saddr, config) and shutdown().
   • Decoupled TCP connection acceptance from SSL handshaking by introducing accept_without_handshake() and accept_handshake(). This allows V's concurrent applications (like veb) to accept sockets on the main thread and perform slow TLS handshakes concurrently in spawned coroutines, avoiding blocking the main event loop.

2. net.ssl:

   • Exported SSLListener and new_ssl_listener dynamically in both ssl_d_use_openssl.v and ssl_notd_use_openssl.v to keep the unified wrapper layer consistent.

3. veb:

   • Refactored vlib/veb/ssl_d_use_openssl.v to utilize the new openssl.SSLListener. When compiled with -d use_openssl, veb now properly runs HTTPS servers using OpenSSL while completely excluding any link dependencies to mbedtls.
   • Enabled concurrent handshaking in spawned connection handlers using accept_handshake().

4. Tests:

   • Added vlib/net/openssl/openssl_sslconn_shutdown_does_not_panic_test.v to verify OpenSSL server binding, client dialing, handshaking, and clean connection shutdowns without panicking or double-freeing.
   • Refactored the old ssl_test.v into two platform-specific/define-specific test files: ssl_test_d_use_openssl.v (for testing OpenSSL HTTPS) and ssl_test_notd_use_openssl.v (for testing MbedTLS HTTPS) to avoid unknown module compiler issues.

Checklist

• I have formatted my V files with v fmt -w.
• I have added standard V doc comments for all new public functions and structures.
• All new and existing tests pass.

Tests Run

# 1. OpenSSL server & client handshake and shutdown test
./v vlib/net/openssl/openssl_sslconn_shutdown_does_not_panic_test.v

# 2. Veb HTTPS integration test under OpenSSL mode
./v -d use_openssl vlib/veb/tests/ssl_test_d_use_openssl.v

# 3. Veb HTTPS integration test under default MbedTLS mode
./v vlib/veb/tests/ssl_test_notd_use_openssl.v

All tests passed successfully on local machine.

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