Fix some Clang -Wstring-conversion warnings

Some false positives, but some flagged faluty asserts
where the ! was inside the string instead of outside.
curl: Use Windows SSL/TLS native implementation

On Windows, when CMAKE_USE_OPENSSL is OFF, use the OS implementation.
This will allow the OS-configured CA list to be trusted automatically.
Merge topic 'assert-no-string-conversion' into next

9713a191 Fix some Clang -Wstring-conversion warnings
Merge topic 'curl-winssl' into next

ba8c6632 curl: Use Windows SSL/TLS native implementation