libvncserver: restore AF_UNIX client hosts

[marcofortina]

Summary

Restore AF_UNIX client host handling for server-side Unix socket connections.

When a client connects through a Unix domain socket, rfbNewTCPOrUDPClient() currently treats the accepted socket like a TCP socket. On IPv6-capable builds, getnameinfo() fails for AF_UNIX and cl->host becomes an empty string. This breaks consumers such as x11vnc that run access checks in newClientHook() before they can repair the host value themselves.

Changes

• Check the result of getpeername() before using the address.
• Detect AF_UNIX sockets on Unix builds and populate cl->host from getsockname().
• Use stable fallback host strings for unnamed or failed AF_UNIX lookups.
• Apply TCP_NODELAY only to TCP clients, not AF_UNIX clients.
• Add a Unix-only regression test that verifies an AF_UNIX client reaches newClientHook() with a non-empty host string.

Validation

Tested with:

cmake -S . -B build-498-patch \
  -DWITH_EXAMPLES=OFF \
  -DWITH_TESTS=ON \
  -DWITH_OPENSSL=OFF \
  -DWITH_GNUTLS=OFF \
  -DWITH_GCRYPT=OFF \
  -DWITH_SDL=OFF \
  -DWITH_GTK=OFF \
  -DWITH_QT=OFF \
  -DWITH_FFMPEG=OFF \
  -DWITH_XCB=OFF \
  -DWITH_LIBSSHTUNNEL=OFF \
  -DWITH_SYSTEMD=OFF \
  -DCMAKE_BUILD_TYPE=Debug

cmake --build build-498-patch --parallel 1
ctest --test-dir build-498-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 6

Notes

Closes #498.