Fix or silence all C compiler warnings discovered with (or without)
`PICKY_COMPILER=ON` (in CMake). This means all warnings showing up in
CI (gcc, clang, MSVS 2013/2015), in local tests on macOS (clang 14) and
Windows cross-builds using gcc (12) and llvm/clang (14/15).
Also fix the expression `nread -= nread` in `sftp_RW_nonblock.c`.
Cherry-picked from: #846Closes#861
This introduces a test suite for libssh2. It runs OpenSSH in a Docker
container because that works well on Windows (via docker-machine) as
well as Linux. Presumably it works on Mac too with docker-machine, but
I've not tested that.
Because the test suite is docker-machine aware, you can also run it
against a cloud provider, for more realistic network testing, by setting
your cloud provider as your active docker machine. The Appveyor CI setup
in this commit does that because Appveyor doesn't support docker
locally.
