libvncclient: add client-aware logging helpers

[marcofortina]

Summary

Adds a non-breaking LibVNCClient logging path that can carry the rfbClient *
context to applications that want client-aware log output.

Preflight

• Identify client from rfbClientLog and rfbClientErr #675 is open and marked as a client-side feature request.
• The maintainer explicitly preferred a separate logging function rather than a breaking change to the existing global rfbClientLog / rfbClientErr callbacks.
• The previous exploratory PR WIP rfbClientLog with client argument #676 was closed after the broad API-changing approach failed CI.

Changes

• Keep the legacy global rfbClientLog and rfbClientErr callbacks unchanged.
• Add optional client-aware callbacks:
  • rfbClientLogWithClient
  • rfbClientErrWithClient
• Add helper entry points:
  • rfbClientLogEx(rfbClient *client, ...)
  • rfbClientErrEx(rfbClient *client, ...)
• If no client-aware callback is installed, the helpers fall back to the legacy callbacks.
• Convert LibVNCClient call sites that already have an rfbClient * in scope to use the new helpers, including:
  • core connection/session logging in rfbclient.c
  • socket read/write logging paths
  • SASL authentication logging
  • TLS/OpenSSL and TLS/GnuTLS client paths
  • Tight, TRLE, Ultra, Zlib and ZRLE decoder paths
  • framebuffer/viewer helper paths with client context
• Keep legacy logging only for helpers that do not have an rfbClient * context, such as listen/socket utility helpers, OpenSSL global initialization helpers, and the public PrintPixelFormat() compatibility entry point.
• Add a regression test that covers both client-aware callbacks and legacy fallback behavior.

Validation

• git diff --check
• CMake build with tests enabled and optional desktop/TLS dependencies disabled
• CMake build with OpenSSL enabled
• CMake build with SASL enabled
• CMake build with GnuTLS + Libgcrypt enabled, matching the failing CI profile
• ctest --test-dir build-675-patch --output-on-failure
• ctest --test-dir build-675-openssl --output-on-failure
• ctest --test-dir build-675-sasl --output-on-failure
• cmake -DWITH_OPENSSL=OFF -DWITH_GNUTLS=ON -DWITH_GCRYPT=ON -DWITH_LIBVNCSERVER=OFF ..
• Final grep audit confirmed that remaining legacy rfbClientLog / rfbClientErr call sites are limited to helpers without rfbClient * context plus examples/tests.

Result:

• default/no-TLS: 100% tests passed, 0 tests failed out of 5
• OpenSSL: 100% tests passed, 0 tests failed out of 5
• SASL: 100% tests passed, 0 tests failed out of 5
• GnuTLS + Libgcrypt client-only CI profile: build passes

Notes

This intentionally avoids changing the existing rfbClientLogProc ABI/API. It gives applications a new opt-in path for client-aware logging while preserving the legacy global callbacks for existing users.

Closes #675.