libvncclient: add FileTransfer client support

[marcofortina]

Summary

Adds initial LibVNCClient support for the legacy UltraVNC/TightVNC-1.3.x rfbFileTransfer message path and provides a working client-side example.

Changes

• Add a HandleFileTransfer callback to rfbClient.
• Add FileTransferSend() for sending legacy rfbFileTransfer messages from LibVNCClient applications.
• Parse incoming legacy rfbFileTransfer messages and dispatch them to the callback.
• Consume the legacy file-download header high-size field so subsequent file-transfer messages remain aligned.
• Add examples/client/filetransfer.c with:
  • file-transfer access request
  • remote directory listing
  • remote file download
• Build the new client example when examples are enabled.
• Add regression tests for the callback and send helper.

Validation

General build and tests:

cmake -S . -B build-83-patch \
  -DWITH_EXAMPLES=ON \
  -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-83-patch --parallel 1
ctest --test-dir build-83-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 6

Runtime smoke against the existing server-side file-transfer example:

mkdir -p /tmp/libvnc-ft-smoke
printf 'hello file transfer\n' > /tmp/libvnc-ft-smoke/source.txt

./build-83-patch/examples/server/filetransfer \
  -rfbport 5997 \
  -rfbportv6 5998 \
  -permitfiletransfer

./build-83-patch/examples/client/filetransfer \
  -ft-download C:/tmp/libvnc-ft-smoke/source.txt \
  /tmp/libvnc-ft-smoke/downloaded.txt \
  127.0.0.1:97

cmp /tmp/libvnc-ft-smoke/source.txt /tmp/libvnc-ft-smoke/downloaded.txt

./build-83-patch/examples/client/filetransfer \
  -ft-list C:/tmp/libvnc-ft-smoke \
  127.0.0.1:97

Observed:

Downloaded 20/20 bytes to /tmp/libvnc-ft-smoke/downloaded.txt
Directory: C:/tmp/libvnc-ft-smoke
  ..
  source.txt
  downloaded.txt

FileTransfer client smoke

Validated against examples/server/filetransfer with legacy TightVNC file transfer enabled:

• server: examples/server/filetransfer -permitfiletransfer
• client: examples/client/filetransfer
• downloaded C:/tmp/libvnc-ft-smoke/source.txt to /tmp/libvnc-ft-smoke/downloaded.txt
• verified downloaded content with cmp
• listed C:/tmp/libvnc-ft-smoke and saw source.txt/downloaded.txt

Result: PASS.

Notes

This PR implements the legacy rfbFileTransfer client-side path. TightVNC-2.x file transfer uses a separate protocol extension and remains out of scope for this PR.

Closes #83.

[Neustradamus]

Good job!