cmake: support OpenSSL 3 with older CMake

[zhanglistar]

Summary

Add a project-local FindOpenSSL.cmake module so QFS can find OpenSSL 3 when
building with older CMake versions.

CMake 3.5 cannot parse the OpenSSL 3 version macros in opensslv.h, which
can cause find_package(OpenSSL) to fail even when OPENSSL_ROOT_DIR points
to a valid OpenSSL 3 installation.

The new module supports both version formats:

• OpenSSL 3: OPENSSL_VERSION_MAJOR, OPENSSL_VERSION_MINOR,
  OPENSSL_VERSION_PATCH
• OpenSSL 1.x: OPENSSL_VERSION_NUMBER

It also respects OPENSSL_ROOT_DIR first, which helps avoid mixing headers
and libraries from different OpenSSL installations.

Test

Tested on Ubuntu 16.04 with CMake 3.5.1 and OpenSSL 3.0.4 installed under / usr/local:

cd bld
cmake -DOPENSSL_ROOT_DIR=/usr/local ..
make -j2